Avatars are images that appear under your name whenever you post a message on the NeoPets Chat Boards (NeoBoards).They basically represent you. There is one avatar for every species (except the Ruki and Yurble), the six basic Faeries (water, fire, earth, dark, light ,air), and the "Lumpy" Christmas Rock are automatically yours. However frome time to time NeoPets may add additional deafult avatars.
To change your avatar, select "Chat Preferences" at the top of the NeoBoards page or to go there directly. Trophies and Avatars
The number of secret avatars you have found can be seen on your user look-up page,and the seventeen users with the most secret avatars receive trophies.(trophies shown below)
NeoNewsNow has a guide with avatar solutions here.