From what I am finding out in my research on the subject of covers-
A traditional publisher would provide your cover art, and I am sure they could take what you have as the theme -- if it is in their estimate that it would help sell your book.
If you are self publishing, or have a smaller publishing house...then you may definitely provide your own cover, in which case you will want to format the art to a standard book aspect ratio.
On the cover itself, I like it. The font is not doing it justice, but I like the glow in the woods, the effect works well. The glow does not appear to be coming from the sword to me, but from the woods in the background.