I'm a fan of Inkscape, and I'm glad to see so many people using Inkscape here too. Since Inkscape is extemely well suited for making Order of the Stick avatars, I've written up a guide on how I made my current avatar.
Learning Inkscape - An Order of the Stick Avatar Guide
Here's the avatar I step through how to make in the guide:
If there's anything unclear, please post here and I'll try to fix it. I'm also happy to hear suggestions for how to improve the guide - it currently finishes quite abruptly and I'm still thinking of what to add next.
Hope this is useful, and that you have fun with Inkscape!