Strange one, I also tried to do same before and couldn't. There doesn't seem to be an extension like jpg. Well there is, but then you get numbers coming after it, such as viewing your avatar. Because it doesn't end with JPG, instead numbers after it. Doesn't get displayed on another forum if you try using IMG tags.
Good in one way because it helps stop hotlinking, but odd none the less how it works. I don't understand it either.
The numbers correspond to the UNIX timestamp of when you uploaded the avatar. It'll change next time you upload one.
It prevents your browser cache from showing the cached image instead of the real one when you update and shouldn't affect images, but if it does just remove the question mark and the following numbers, it'll do no harm.