Katvixen,
I do believe there is a plugin that does this. Check the
http://codex.wordpress.org/Main_Page wordpress codex.
If you cant find one there check the wordpress support forum.
I'm pretty sure that there are plenty of plugins that do this, especially sites for multiple posters, and I've seen ones for a different avatar/iamge for each category as well. if you still cant find anything feel free to icq me.