if you can find a JASC Animation Workshop, that would be a great way to start doing your own customizing. As as it was said already, GIMP, Photoshop, and all those other image editors can be used to
personally, i think http://www.allavatars.com/index.php makes a decent start up for an avatar that is not like most(most are not fancy as you say, but i think they are a sufficient, at least for the time being, brake from regular avatars.