or what i do instead, make sure you have a appropriate sized picture for your signature then.
go to user CP
Edit Avatar
click the little button " use my custom avatar "
Browse, find your picture,
oops thats for your avatar..
for a signature just
alt, control, print screen. or for a laptop for " me " fn and press print screen.
make your picture by going into paint, photo shop ect,
erase what you dont want.
resize it by making the little arrow thing, fit the picture
save.
go to a HOST website for hosting pictures photobucket, imageshack, etc
upload the picture, then get the IMG url for it and put it in your signature.
click it, then upload