What has worked for me:
As regards online promotion for me it's working this method I developed over the years:
From my logs:
First Post in Blogs, you need blogs, Tumblr, wordpress, etc.
Link blogs to fb stumble upon Digg Pinterest Twitter
Create profiles in your niche related sites and link to your work.
Link T-Shirts on shop to fb stumble upon Digg Pinterest Twitter weHeart it, svpply, My artpage Google+
Link Pinterest to fb stumble upon twitter
Tweet Often Create more than one account, one for you as an artist and some with keywords in its username ;-)
Comment on ... Blogs with tees
links in about.me
PINTEREST PINTEREST PINTEREST !!! (Create boards related to your brand or niche and you'll get followers if you pin often and good stuff, and add descriptions to your work saying it's available for sale!)
and everywhere you can add many tags, think of what the customer would type in the search box, and what combination of long keywords you should use, for instance I use "cool vector retro vintage" a lot, cos in Redbubble those are filters they use to showcase work ;-)
Let me know if you need extra help!
I love online marketing and helping with it, it's not easy and I had nobody to teach me, so if I can be of any help....