Doesn't it tell you how much it can fill? If the file is larger it might just resize it automaticly :(
Well the upload req might give a hint...I dunno...but according to this site; Note: The maximum size of your custom image is 500 by 100 pixels or 9.8 KB (whichever is smaller). ...and when I looked your sig was about 11kb...so that might be it...as the pixel size is only 343 x 82 pixels atm...
I just doubled the maximum size for signatures. Please try again, and see if that did the trick? Thanks :)