i think you just need to find a image hoster that supports that type of file. i have never tried to have a .png sig though, so i dunno how GR will effect it.
just upload it on imageshack put it in yout sig and get rid of the url bits so it has the link inside brackets. png is better quality that jpeg by far, I use png myself.
When making the sig it has to be transparent, then you Save As -> *name*.png. Then you open up imageshack.us and upload it there. This is in the wrong section so you'll get a fine FYI. :| Hope I helped..