When using the Email activity to send an HTML email that contains an image file in the email signature, the image is showing blank with a place holder. Attached to this KB article is also a reproduction video to validate if the issue explained is the same.
This is a Thunderbird issue. The images are passed as cid https://sendgrid.com/blog/embedding-images-emails-facts/
Below is the bug reported with Mozilla.
61815 - Image not shown inline when using "cid:" due to wrong Content-Type: multipart/mixed - Make TB more tolerant to incorrect MIME structure: accept multipart/mixed where multipart/related is required (mozilla.org)
Use any other email client besides Thunderbird like Outlook and the email signature file will show correctly.
Level of Complexity
Add any references to other internal or external articles