Your Outlook signature image is not showing for clients, arrives as an attachment, turns blurry or disappears in dark mode? These problems look random, but they follow precise rules: how the image is inserted, its format, its resolution and the recipient's security settings. This guide starts from the symptom you see and works back to the cause, then to the fix that lasts.
What is the symptom? The diagnosis at a glance
Start by sending a test email to an external address, ideally read on another device. The exact symptom points straight to the cause:
| Symptom | Likely cause | Fix |
|---|---|---|
| Empty frame or red X, with a prompt to download pictures | Recipient's default blocking of external images | Alt text; the recipient approves the sender |
| Logo received as an attachment, especially on mobile | Image embedded in the message (pasted or inserted) | Image hosted over HTTPS |
| Logo too large, too small or blurry | Image resolution above 96 DPI; dimensions not set | Width and height set in pixels |
| No image at all, text only | Message written in plain text | Switch the message format to HTML |
| Logo missing in Outlook for Windows, visible elsewhere | SVG image, which the Word rendering engine does not display | Convert to PNG or JPG |
| Logo invisible or inside a white box in dark mode | Dark logo on a transparent background | Opaque background or a version that reads on dark backgrounds |
Why doesn't my signature logo show up for the recipient?
According to Microsoft, Outlook is configured by default to block automatic picture downloads from the Internet. The goal is to protect privacy — a remote image can act as a tracking pixel that tells the sender the email was opened — and to avoid unwanted content. Until the recipient clicks "Download Pictures" or adds the sender to their Safe Senders list, the logo stays an empty frame.
This behaviour is set on the recipient's side, not the sender's. What you control is what the recipient sees in the meantime: alt text (the alt attribute) carrying the organization's name keeps the signature understandable, whether the image is blocked or not.
Two special cases are handled on the IT side:
- Blocking enforced by policy. If the
BlockHTTPimagesregistry value is set to 1, often through GPO, classic Outlook refuses to display any image referenced by a web address, even after clicking "Download Pictures". Microsoft documents this case under the error "The linked image cannot be displayed". - Known issue from May 2026. After the May 12, 2026 updates, classic Outlook stopped displaying some images referenced with
cid:, including in signatures. Microsoft released a fix, rolled out no later than the June 9, 2026 updates; replies and forwards written during the issue may have lost the image permanently.
Why does my signature logo show up as an attachment?
Because the image is embedded in the email instead of being hosted. That is what happens when you paste a logo into the signature editor or insert a file from your PC: the image then travels inside the message, as a separate part referenced by a cid: identifier. Several mail clients, especially on mobile, display those parts as attachments — and your logo ends up in the attachment list of every email.
Images encoded directly in the HTML (data:image, known as base64) cause a related problem: they inflate every message and several clients ignore them. Answers on the Microsoft Q&A community advise against both methods for signatures.
The fix is the same in both cases: host the logo on an HTTPS web server and reference it by its full address, for example https://example.com/logo.png.
Why is my signature logo blurry or too large?
Outlook for Windows uses 96 DPI (dots per inch) as its reference resolution. A Microsoft community moderator puts it this way: an image with a higher resolution can be resized and distorted by Outlook. A logo exported at 300 DPI from a design tool can therefore display at an unexpected size, or blurry.
Three rules solve the problem:
- Save the logo at 96 DPI, at the pixel dimensions you want.
- Set
widthandheightin pixels on the<img>tag. Outlook ignoresmax-widthandheight:auto. - Prefer a hosted image, whose display size is driven by those HTML attributes rather than by the file's metadata.
The other image weight and accessibility rules are covered in the email signature templates guide.
Why does my logo disappear in dark mode?
In dark mode, classic Outlook for Windows, the new Outlook and Outlook on the web show the message body on a black background. A dark logo saved on a transparent background blends into it, and some transparent logos end up inside a white box. The reader can restore a light background for a message with the sun button, but you cannot count on them doing so.
Two reliable approaches:
- Put the logo on an opaque background (white or brand colour) with a small margin built into the image.
- Pick a version of the logo that reads on both backgrounds, for example with a light outline.
Hosted or embedded image: which should a signature use?
| Criterion | Hosted image (HTTPS URL) | Embedded image (cid or base64) |
|---|---|---|
| Email size | Light: only the address travels | Heavier with every image |
| Shows up as an attachment | No | Often, especially on mobile |
| Displays immediately | After the recipient allows it, if images are blocked | Yes in most cases |
| Size stability | Stable when dimensions are set | Subject to resizing based on resolution |
| Updating the logo | Once, on the server | In every signature |
For an organization, the hosted image is the right choice: it does not clutter attachments, keeps a stable size and is updated in one place. Its only drawback, default blocking, is offset by alt text.
Which format and size should a signature logo use?
- PNG for a flat-colour logo or one with text: sharp and light.
- JPG for a profile photo.
- No SVG: Outlook for Windows uses the Word rendering engine, which does not display SVG images.
- Fixed dimensions in pixels and a small file size: an optimized signature logo weighs a few kilobytes.
- Alt text every time, with the organization's name.
How ATOM Signatures handles signature images
ATOM Signatures applies these rules by design. The logo uploaded in the console is hosted in Canada and referenced by a full HTTPS address: it is never embedded in the message, so it never turns into an attachment. SVG files are rejected on upload and stripped from the signature code, to avoid an invisible logo in Outlook for Windows. The add-in inserts the signature client-side, inside Outlook, and a logo change applies to the whole organization without touching any PC.
Fix a signature logo in six steps
- Send a test email to an external address and note the exact symptom: empty frame, red X, attachment, blurry image or logo invisible in dark mode.
- Check that the message is in HTML format, not plain text.
- Replace any embedded or pasted image with a PNG or JPG image hosted over HTTPS.
- Set the image width and height in pixels in the signature's HTML.
- Add alt text that carries the organization's name.
- Test the rendering in light and dark mode, in Outlook for Windows, on the web and on mobile.
Frequently asked questions
Why doesn't my signature logo show up for the recipient?
Most often, because Outlook blocks automatic picture downloads from the Internet by default. The recipient sees an empty frame or a red X until they download the pictures or add the sender to their Safe Senders list. Alt text on the image keeps the signature readable in the meantime.
Why does my signature logo show up as an attachment?
Because the image is embedded in the message instead of being hosted. An embedded image travels as a part of the email, and several mail clients, especially on mobile, display it as an attachment. The fix is to host the logo on an HTTPS server and reference it by its address.
Why is my signature logo blurry or too large in Outlook?
Outlook for Windows uses 96 DPI as its reference resolution: an image saved at a higher resolution can be resized and distorted. Save the logo at 96 DPI and set its width and height in pixels in the signature's HTML.
Can I use an SVG logo in an Outlook signature?
No, it is not recommended. Outlook for Windows uses the Word rendering engine, which does not display SVG images. Use a PNG for a flat-colour logo, or a JPG for a photo.
Why does my logo disappear in dark mode?
In dark mode, Outlook shows the message on a black background. A dark logo on a transparent background then becomes almost invisible, and some transparent logos get a white box. Put the logo on an opaque background with some padding, or pick a version that reads well on both light and dark backgrounds.
A logo that displays everywhere, with no attachment. ATOM Signatures hosts your images in Canada and inserts the same signature in Outlook for Windows, web, Mac, iPhone and Android. $1 per user per month, plus taxes, with a 7-day free trial.
Go further: professional email signature templates, signatures in the new Outlook and the best email signature software in Canada.
Sources
- Microsoft Support — Block or unblock automatic picture downloads in classic Outlook email messages: support.microsoft.com
- Microsoft Support — Images are not displayed in emails, newsletters, or signatures in classic Outlook (updated June 5, 2026): support.microsoft.com
- Microsoft Learn — "The linked image cannot be displayed" error (BlockHTTPimages): learn.microsoft.com
- Microsoft Support — Dark Mode in Outlook: support.microsoft.com
- Microsoft Q&A (community forum) — 96 DPI recommended for signature images: learn.microsoft.com
Sources accessed September 17, 2026.