I remember this pic! It's how I found you on DeviantArt, really a great piece. It sucks that you haven't been given due credit. Have you considered writing in to those blogs/sites/magazines that you've seen it used?
yep, no reply from them... what can you do
Depends on how much it means to you. You could a cease-and-desist or a take-down letter, both of which could technically be reinforced by a court of law.If you need some more info, this article is a great starting point and resources: http://escapefromillustrationisland.com/2010/04/09/what-to-do-when-your-artwork-is-used-without-your-permission/