Need help changing avatar, OH says it must be certain conditions?

New picture is showing but it states something about needing to PGF or something like that and 100X100 pixels. I use Picasa2 for my photos and don't know how to change resolutions or anything they are asking of me. Can anyone help? Thanks for any help!

We greatly appreciate your interest in helping us build our Q&A database. To discourage vandals from posting garbage, however, we require people to register before posting.