Howdy! I've been supporting myself financially as an online freelance artist for about 7 years. The one thing I noticed about being a freelance artist on the internet is that it's very difficult to make a living doing just one thing. I make a pretty decent earning using several different avenues on the net, including selling my graphic designs through my website as well as taking custom art commissions as well. My largest income earner to date are my children's book illustration requests. I work on about 10 - 20 story books a month, sometimes more. I don't work for any company in particular, it's just myself promoting the crap out of myself.
I recommend signing up with all of the major social media sites (twitter, facebook, google+, tumblr, instagram) as well as a few other lesser mentioned sites (pinterest, linked, 4 square, digg among many others) and just promoting the heck out of yourself at any given chance. You should also be plugging your portfolio links in as many places on the net as permissible (forum signature, email signature, a link from blog post comments) as well as using some free classified ad sites such as craigslist among many more.
And don't forget about business cards! I go to conventions pretty often and I give out business cards like candy. Always have plenty on hand !
Just be consistent is all! Try to be active on all of your profiles everyday and you will definitely see results!