Well, my opinion would change depending on what you were going for. It seems you just wanted an area in the game and slapped on a render. Compared to some other first time signatures, I give it a thumbs up, although I like my signatures to have more effects. For example, I didn't make my current avatar's background. It was some stock image I found through Google. With some slight edits, I made it brighter, made it blend in more with the background, did some smudging with the wolves, and I was done, with an improved image.
I think it would be better to make a background for a signature. Either that or edit a stock like crazy. That will give you some cool effects that really make the sig noticeable. That's what I do for everything I make. I would recommend doing some Google searches on "GIMP signature tutorials" to find some nice additions to future graphics, if you plan to make more, that is.