This document explains some of the reasons to leave images (e.g. JPGs, GIFs, PNGs) out of your email signature.
There may be compelling reasons to add an image, such as a logo or stylized quote, to an email signature. However, adding an image to an email signature may result in the email message with the signature being viewed as suspicious or spam-like. The delivery of the message may be delayed or rejected by recipient email systems and/or their spam scanners. This has the potential to degrade the reputation of UW-Madison's email system and its senders, and thus the UW brand, over time.
Both campus and external email system administrators may have other reasons, unrelated to security, for delaying or rejecting an email message with a signature image, such as their own email message and attachment size limits.
Because of the above reasons, the inclusion of images in email signatures is not recommended.
For additional reasons that you, as the sender of an email, may want to reconsider including an image in your signature, please see the example article below (not affiliated with or endorsed by the UW):
Some best practices for email signatures in general can be found in the following article (also not affiliated with or endorsed by the UW):