Not nearly as weird, Mike. (IMG:
style_emoticons/default/smile.gif) Something similar happened to me an year or so back, and various images of various sizes and dimensions did not go through. All were withing the acceptable limits. I dashed it all, and went ahead with my current signature (the day Maldini retired), which surprisingly was accepted.
Luckily, for a guy who changed signatures almost once a week, I've decided to stick with this one for all eternity. But anyway, it would be good if you can spare a few minutes and look into things, maybe there is some sneaky setting in the Admin panel that's causing the trouble!?