I do not have a complete answer for you, but maybe some hints at what you are looking at.
If you pull up one of the utilities that allows you to look at the MUL files in game, the animations are in one of those. I do not remember which file that is, because I looked at it using UO Fiddler version... 4.6? It is an older version of that program that I had than the most recent.
The animations are based on artwork for each frame, and the frames are cycled by the client during actions. You can pull up a base picture out of the animation for an entity (human, elf, gargoyle) and scroll through each of the animations for that character.
I am not a very good digital artist, so I probably wouldn't try myself - because I would make something that looks like an unshaded shape.
![Wink ;)](http://www.scriptuo.com/Smileys/yarex2/wink.gif)
It is possible to create artwork and replace the animations in the file, then save it back.
That is a very simplistic overview that may get you down the right path...
There are also open source clients that allow doing things like replacing artwork, but I believe that is more global changes (replace all trees with flowers...).
There is a section of this site dedicated to various client mods, maybe something there will help you learn what you are after:
http://www.scriptuo.com/index.php?board=135.0