I've done some of my own in Paint.Net. I'm also lucky enough in that my fiance is an amateur graphic artist and does amazing work. (Seriously, I know I'm obligated to say that, but even if I wasn't I'd still say it. She's really quite good.)
EDIT: I forgot to put my point in, sorry. Paint.net isn't that hard to use (and it's free), and there are a lot of tutorials and how tos on the internet and if I can generate a salable cover (they are selling slowly but fairly steadily. Not enough to live off of yet) then I reckon most anyone can. I'd also note, my covers are all for shorts and I refuse to pay $100 or even $50 for the cover of a short story. A collection, quite possibly, a novel, yes if I had to, but for a short story? Nope. Not going to happen.
My. .02