Avatars:

For SOF members: Use your i game character avatar, dimensions exact the same you see above your character's sheet. Cut the name and you're good to go.

For visitors/ guests/ candidates: Make sure your avatar won't reveal your ooc identity ( for example, used at several forums or representative of your main character in game )

Signatures:

SOF members: Not applicable.

For Visitors/guests/ candidates the same rule applies as for the avatar: nothing that reveals your identity.