Outlook HTML email signature - recipient images are blank

Yes—what is shown is consistent with Outlook image rendering behavior, and the failure can happen for different reasons depending on how the images are referenced.

Key points:

1. Externally referenced images can fail to display Classic Outlook can block or fail to render linked images. Microsoft documents cases where invalid IMG SRC values cause Outlook to drop the image during HTML import, and file: image sources are treated as not secure.
2. CID-based images are also not guaranteed in every scenario Microsoft has documented a classic Outlook issue where images referenced with cid: in message source were not rendered after certain updates. That means CID embedding does not guarantee that images will always display.
3. Rich Text and HTML formatting matter In classic Outlook, if a message is sent as Rich Text, attachments can appear in the message body. For image-heavy signatures, HTML format is the safer format.
4. There have been Outlook-side bugs affecting signature images Microsoft has documented multiple Outlook issues where images in signatures, newsletters, or embedded content showed as blank or red X, including cases fixed by later builds.

What this means for the scenario:

• If the signature uses external or local file references, the recipient may see blanks or red X if Outlook treats the source as invalid or insecure.
• If the signature uses CID/content-ID images, that can help avoid dependence on external image download, but it still does not fully bypass Outlook rendering issues because classic Outlook has had known CID-related display bugs.
• The problem is therefore not limited to the recipient’s mail server. It can also be caused by Outlook client behavior.

Recommended setup based on the documented behavior:

1. Use HTML message format for the signature emails.
2. Do not use local or UNC/file-based image paths in the signature HTML.
3. If manually inserting pictures in Outlook, that aligns with Outlook’s normal signature workflow and can create embedded image references, but there is no documented method here that guarantees Outlook will automatically convert every HTML signature image into a reliable embedded image at send time.
4. If the issue is specifically the classic Outlook error “The linked image cannot be displayed”, Microsoft documents a registry-based fix by removing BlockHTTPimages.

Important: modifying the registry is risky. Back up the registry first. The documented steps are:

• Exit Outlook.
• Open Registry Editor (regedit.exe).
• Check one of these locations for BlockHTTPimages:
  • HKEY_CURRENT_USER\Software\Microsoft\Office\x.0\Common
  • HKEY_CURRENT_USER\Software\policies\Microsoft\Office\x.0\Common
• Delete the BlockHTTPimages value.
• Restart Outlook.

If the value is under the Policies hive, it may be enforced by Group Policy and will return unless the policy is changed.

On whether to use the Outlook signature editor or edit the signature files directly:

• The available documentation here supports that Outlook’s built-in signature workflow is the normal path for inserting pictures into signatures.
• There is no supported guidance here stating that editing the signature HTML files directly is better or more reliable for automatic image embedding.
• Based on the documented Outlook behavior, using the signature editor inside Outlook is the safer approach.

References:

• Classic Outlook shows error "Contacting the server for information"
• Images are not displayed in emails, newsletters, or signatures in classic Outlook
• Embedded images in Rich Text formatted emails or fields are being blanked out or misaligned
• The Linked image cannot be displayed error in an Outlook email message
• Troubleshoot attachment problems in Outlook for Windows