go to user CP (at the top) they click avatar (in the left side)...and if you want to do a signature, click signature (same place as avatar) then upload the image and insert it in the sig
Or if u have a pc just open up the pic in paint an then goto tools an then strech/scew an then just make the percent numbers smaller an it'll shrink the pic then save as is an rename it. That's what I do an it only takes a few seconds to do