As a one-person publisher I have to perform a lot of divergent functions. One piece of advice from an award winning artist / author was to avoid getting a cover on the site linked below. If you take a look, you'll see why.
http://lousybookcovers.com/?paged=7
I also realize with all the duties I have to juggle on a given day, I'll likely never be able to create a model from the following illustration of a prototherian (egg-laying mammalian) character from the novel I just released.
But I can take my basic android to do a lead android character from the last book in a series I'm working. If you looked at the link from "Lousy Book Covers" perhaps it makes more sense why I strive to do my best at creating a cover element. If my one week of experience is a guide, I don't need to quit my day job.