Here’s an article by William “Proton” Vaughan in Computer Graphics World on how to market your artwork online:
Get Online, Get Noticed
When my career in 3D was beginning, I set out to meet others who, like me, shared a passion for art and animation and wanted to make a career of their craft. What I found was a flourishing art community that exists on the Internet–many forums and online communities where talented artists and animators share ideas, offer feedback and critique, and help others with tools and techniques to work around creative design and animation challenges. Not only are these portals excellent information resources, they are a way to get your art in front of literally thousands of people. The only investment you have to make is your time, but with a little perseverance, the commitment can really pay off.
He goes on to list some online communities (I del.icio.us’ed them for ya :P)
Read the article for tips on how to behave in online art communities.