I think it’s a great image, but a few small touch-ups could make it a wonderful image. Like above mentioned, perhaps making the wizards pose a little more dramatic, or giving some indication that he is casting a spell (if that is what he’d doing of course) would help the image a lot.
Two more small things that I picked up right away.
Firstly, the dragon’s eyes, I can see where they’re looking if I look closely enough, (the small lighter-red dots in the eyes I think are what’s looking at the wizard), but for a book cover something like this has to leap out at the viewer when they first see it. Maybe making the irises more obvious, brighter red or even adding a slight glow too…
And lastly I think the lower half of the image is much more detailed and generally nicer than the top half, or to be specific, nicer than the pillar in the top half. It seems you’ve put a great amount of effort into the lower portion, like the stairs, trees etc, and the top looks rushed… Maybe giving the pillar more detail, sharpening it up, giving it moss, cracks and clearer runes.
Otherwise this is a really nice image, for a book cover it could be very gripping, nice job and stick at it!