Upload your file on some uploading site, I recommend www.imageshack.us
Then press the insert image button (Yellow with some mountains and a sun) and copy the link that you got from imageshack, and that's it, it should work.
Ghan has said he has fixed this. Monovertex please confirm this fix. This was only a problem with people that had signatures in the upper levels like not the special members but the respected members.