Just a side note, once upon a time (before Nov 2011), signature images are allowed. 
https://bitcointalk.org/index.php?topic=51077.0
https://bitcointalk.org/index.php?topic=51077.0
Wow, I forgot how bad the user response was after that. Now hardly anyone even remembers it...
There are also security problems: Images can be used to execute cookie stuffing and cross-site request forgery attacks; they use a lot of bandwidth; and they prevent bitcointalk.org's HTTPS from appearing totally "kosher" to browsers. These issues could be solved by hosting all images at bitcointalk.org, but this isn't worth the trouble.
I guess I decided that it was worth the trouble after all.
Wouldn't it be possible to use the forum image proxy like we do for posted images , for signatures and avatars ?
I guess there would be more images posted than avatars or signatures, so it wouldn't put too much additional strain on the servers.