I just had a quick look at the sites, and found many of the covers they display as just ugly and others as very hard to read due to the text blending into the image way too much. Also, I recognise some of the images used as ones I've seen else where for either a dollar or two or being free to use. Thanks for the links, but I see nothing there I'd care to use.
That's why I said I only liked the one site, which also had the most professional images with some of the lowest prices. As for not having the sources, I've worked with that before. You basically take the image they give you (without the titles) and create your own source file with it (though that means you have to purchase the fonts they use.
In the one book I'm looking to have them do (because I've been unhappy with the images I've found myself), I'd use my own fonts anyway, since it's a sequel to an existing series.
I haven't yet talked to the site's owner, but I asked him how he'd prefer to handle different aspects (like passing me the source file, my passing him my own fonts, or his simply giving me the source image and my editing the file myself). I see what he says.
Still, for those without a background in the graphic arts, it's a reasonable solution, and much better than the traditional $500 most people charge.
That said, I know an author who augments her income by designing book covers. She does a decent job and only charges $25 + expenses (cost of the image, etc.). I can always give you her number if you're interested.
I still prefer my own images, but when you can't quite find the perfect image, this is a nice alternative.