Those lil' pics are called "Avatars". Here are a few meanings:

1) The incarnation of a Hindu deity, especially Vishnu, in human or animal form.

2) An embodiment, as of a quality or concept; an archetype: the very avatar of cunning.

3) A temporary manifestation or aspect of a continuing entity: occultism in its present avatar.

Most BBS' let you put a small avitar by adding it in your profile. Sometimes having an Avitar must be earned.

Not sure how to get one on this site. Read the FAQ's perhaps.