Hi,
Right now, Rei cannot display the OpenSim avatar appearance.
This topic is very high on the wishlist for Rei, and the community has started to investigate ways to introduce customization. There is now a method to dynamically change textures using javascript. Depending on your needs, this might be enough for you, but unfortunately you will have to wait for a more complete avatar solution.
Technically speaking, you will find a fixed avatar mesh that is displayed for standard OpenSim grids. You can find the name of this mesh by looking at AvatarManager.cs (it's actually 70101ee9-888d-5bd6-371f-925c9b668ce3, so if you want to play around with "custom" avatars locally, you can try to replace the underlying mesh that is placed in the user cache's models folder, and maybe you could quickly change the source to have a sort of avatar-list for your users)
In any case, you will definitely see custom avatar support in Rei. Of course any suggestion on how to make custom avatars happen is always more than welcome.
Zaki