I presume you want a picture under your name on the left side of your posted posts?
If so, look at the darker blue heading line just under the Automotive Forums>Coffee Break>COMPLETELY off-topic.....etc. line.
Click User CP, in Control panel goto edit avatar.
Use the download from computer text-line.
** I found it best to have whatever pic. you want to use to already be in either My Documents or My Pictures. Use the one you want, and enter it in.
See if that works.
It's been a while since I put my attack pig, but I don't recall anything needing to be a certain size or anything to bring it in.