When I started self-publishing two years ago, I tried a number of different things and wasn't happy with any of them, and ended up deciding to do my own. But I'm enough of a designer that I can do that. (I also have the software and equipment.)
My big issue is this: you have to be able to match your genre's style. The audience will use your cover style to judge the genre, and probably the "flavor" of your book. Since I always write cross genre, I get more flexibility than most people.
Thriller writers have it easiest: the standard out there is big type. You can almost do anything else with the images, or have no image at all, if the title and author name dominate the design. But you do have to have a sense of typography. Hard SF can get away with this one too. Literature and some historical versions of various genres can get away with public domain classic art. Chicklit needs sassy cartoons, Romance needs torsos.
Look at what is being published in your genre. Look at authors who have the audience you want. Do their covers use photos, or illustration? Silhouettes, fine painting, moody lighting? Does it depict people or settings or abstract items?
When you're done with that, you'll know what the options for your genre is.
The good news is.... even if you write in a genre which has demanding and expensive standards, remember that there are hundreds of authors out there in the same boat, and many of them are established names who are looking to self-publish their backlists -- and they don't have money either. They're going to sell their books based on their names, and the cheaper style of cover they choose will bring that style into the fold for the genre.
If you decide to go with stock images, Shutterstock has great stuff, but they also have licenses that limit what you do. If your book becomes wildly successful, or you use the image as a template for a series, you may run up against the limits of how many "impressions" you can sell before you have to pay them more money. (Unlikely but it's a hassle I don't want to deal with.)
Fotolia considers book covers to be like ads or web graphics, and doesn't limit the sales count. (At least that's what they told me via email -- the sales count limit is for items where the customer is buying it for the image -- such as T-shirts.)
I'm generally too fussy to work with anything but the most expensive artists (and I can't afford them, so that's why I do my own). You do get what you pay for, and if you want to have redos, you need to compensate the artist for her time. Young artists are not as good at listening as experienced artists.
And as Wildbow said -- think about where the cover will be seen. I worry less about how it will look in grayscale than how it will look in a tiny thumbnail version.