It is yours. Look at the cloud formation to the left on the planet. They form the same type of sideways "V" shape as the ones on yours. The sun flare is in the same spot. The planet gradients from blue to green on the right as well. I would definitely say this is yours.
I think it is your Horizon's background, however they made it a little darker at the bottom of it... I also think your Nova One [link] might be under Thor's arm too
Copyright belongs to the creator of the image as soon as it is created. You are able to follow it up in court.. however it may not be worth your time/money to do so, or it might be something you wish to do. That would be something you would weigh yourself.
Thank God it was only fan art, I have had 2 companies use my art without permission and had to go through weeks of trying to get compensation... But I do have to admit your background would make a fantastic one for a real Avengers poster!
My work gets stolen because people take it from deviantart and post it all over the internet. My fault for making wallpapers and not marking them thoroughly because people complained about walls having watermark and sigs etc... Now I mark them better. Once I contact the company using it they explain how they either bought it thinking it legal or thought it NASA and they then paid me. There has never been any question by them that the orig work is mine since I send them links to my works on dA and offer them the original files as proof (which they have never asked for,lol) On the positive side, if people had not taking them and posted them all over the internet I doubt seriously that I would have been "found" and got the work that I now have. So it is a double edged sword. I doubt seriously that copyrighting them with the US Copyright office would do any good as it would cost me more money than I have and those who wish to steal will anyway, copyrighted or not.
I think it's probably the most-stolen space scene in existence. Or at least rivalling `taenaron's "Rise of a Planet" piece. Or even my own "Golden gates of Hell" (which I fear to search for online...seriously, finding out about this stuff makes me feel physically sick and I don't have time for that kind of stress right now )
At least this wasn't official. Then again you could have sued them for a lot if it were