Well, this is going to be a very challenging project!
Maybe you've already seen it, but I'm working on a similarly challenging project myself, and I'm posting works in progress here
viewtopic.php?f=9&t=5862There's not really much I can say besides my comments in
this message on Shading. Unless you want to do some kind of stylized thing, duplicating it exactly will be very difficult. Maybe you can kind of look at it as a learning excercise, because that's really the only way to learn (you know, practice, practice, practice!) You'll definitely want to use layers, because you'll be challenging Inkscape as well as your computer's capabilities, with the sheer number of nodes alone, not to mention blurs, filters, gradients, etc. Unless you have massive amounts of RAM and processor, Inkscape will probably crash many times.
I don't mean to discourage you, and I don't mean to say it can't be done with Inkscape. There have been some spectacular photorealistic images made with Inkscape (a few are posted in Finished Work forum) by some incredibly talented people. I just want you to have all the facts, based on my own experience learning Inkscape. Maybe you could search out some appropriate tutorials that cover shading, etc., to get some idea what all is possible?
All that said, I did just notice your topic title says "avatar". I don't know if you mean that because of the name of the movie, or if you want to make an avatar for a forum? If you want a forum avatar, that will be much easier, although with it's own challenges as well. Before I launch into comments about making very small images, I'll wait for you to confirm if that's what you want to do.
It looks like you have made a good start though! I'd say just keep on working. And I hope you will post screen shots, as you continue to work on it. It will be both interesting and informative to watch it develop.
Edit -- just happened to come across this tut -- covers some shading, shadows --
http://www.libregraphicsworld.org/artic ... icle_id=12