The following rules are no longer true and are kept only for historical reference. Please see this topic for the updated version.
I understand armory-generated signatures are pretty popular, but please mind the other users (slow connection, small-screen devices...).
More than one image per signature is no longer allowed. "Giant" images in signatures are not acceptable either - the usual size is 64px height so please do not go too high over that. In any case, do not go over 100px.
100px max height. (with rare exceptions)
500px max width.
Animated GIFs are not allowed.
Rotating (changes on refresh) images are OK if they aren't heavy.
PNG preferred, or JPG for very detailed ones. GIF acceptable but not recommended.
Chaud: I will be removing massively violating signatures, and the only thing left will be a mess and your image URL.