As a game background, i think it may have too much contrast and saturation that would make sprites hard to make out. As a lone piece (such as an image for an intro or cutscene), it looks very nice.
The scale of the foreground objects makes them look small: the skull part looks like a small sand castle (such as one a kid would make on the beach), the perspective on the sand could contribute to this feeling of small scale.
It's a bit hard to understand the foreground as a whole: it seems to me there's an unfinished bridge on the left, which has no beginning nor end, nor does it cross any chasm. There are some plants under it which may or may not be forests, and it leads to a desert that looks like a sand box. Contrast between the layers and more object clarification/unification would benefit the piece, i think.
The skull's readibility isn't there at all, maybe add some aspects of real skulls so it looks more identifiable (cheekbones, reshaping the eye sockets), while not looking exactly like a skull (like a natural rock formation would).
Despite these factors, i quite like your work!