Chuck,
As an FYI check that topic thread over on vbulletin.com they said that the files ALWAYS are converted to .GIF files and that they follow the pattern of
"sigpicUSER#_IMAGE#"
So for example if user #300 on your site uploaded his 4th signature picture attempt the file would be stored as "sigpic300_4.gif" but ALWAYS as a .gif file, so sounds like you could hardcode that...
