This is a post about cover-art concepts for your book rather than actually sourcing art. Whether you're drawing your cover yourself, using stock art or commissioning an artist, you need to have at least some idea of how you want your cover to look before you get to that stage. You need to be able to tell your artist what you want them to draw, even if your artist is you.
And cover concepts are hard work. Thinking up a great concept involves thinking in a quite un-writerly way; when we think visually normally, we think about the words that will most effectively convey the images in our heads. When you're coming up with a cover concept, you're looking for an image that will convey something about the words in your book. It's all backwards.
As I'm pretty pleased with my cover concepts for 'The Death of John Collins' and 'Bad Romance', I'm going to talk a bit about the processes I used to develop them, as ideas you might want to try if you're struggling. I hope you've seen the 'John Collins' cover and sample I've already blogged.
My process for 'John Collins' (which may be getting a new title, by the by) was to start from what I thought was the strongest selling point of the novel. Because we're talking sci-fi, and looking for something which could be expressed in a very few words, I focused on the time theory which underlies the John Collins story, because this allowed me to make my cover concept 'John Collins destroyed time'. I don't know how rare the idea of time being destroyed is, but I've never seen it before, and it makes for a pretty good line regardless.
Add to this the fact that the title is 'The Death of John Collins' and we've got to mix the destruction of time with a human death. The idea of some sort of damaged timepiece (with the damage being clearly from a human-killing weapon) was an easy one to reach from there, and I played around with various clocks before hitting on an hourglass - which won out ultimately because it's not actually very hard to draw bullet-holes in glass.
So, one way to find a cover concept is to start from a key concept in the novel. This is particularly good if, as I do, you like quite abstract/stylised cover art. Another method, which I'm using with 'Bad Romance', is to take inspiration from a scene in the novel. Some books - the original covers of Anne McCaffrey's 'All the Weyrs of Pern' and Dan Simmons' 'The Fall of Hyperion' are great examples - go as far as just using a painting of a scene, but this is risky (the artist usually gets something wrong; it's always bugged me that the Shrike on Simmons' cover has too few arms).
You don't have to go this literal, though. Lots of sci-fi books thrive on generic scenes of spaceships which could be any one of a number of scenes within the book. You can also go for a less 'scene-like' piece of art, as for example with Alastair Reynolds' books, which tend to have a spaceship from the novel rendered against a simple stylised background (I absolutely love Reynolds' covers - see 'Revelation Space' for a particularly good example).
Picking a scene or image from the content rather than the theme of the book, you still need to pick something which is representative of your theme and of the book as a whole. One of the reasons McCaffrey's 'All the Weyrs' cover is so good is that the scene - dragons in space - is one of the archetypal moments of the plot. A representative cover is honest marketing; and dishonest marketing tends to be a weak strategy in the long term.
So, I need to pick a scene from 'Bad Romance' which is representative of the book and which also has a striking image. Fortunately, the story of 'Bad Romance' includes descriptions of a number of music videos which are loaded with distinctive visuals (he said, modestly - and yes, the book was partly inspired by Lady Gaga, if you hadn't guessed). The story is about pretending to be someone else and about trying to find out who other people are pretending to be, and a couple of the videos described in the book play on this theme with characters wearing various masks.
Here's a very rough concept sketch I knocked up based on a scene from one of the videos (I was going to post the description of the scene, but looking at it, it needs some redrafting-type work):
I scribbled this in a spare minute this morning while the workshop I TA for was quiet, but I think you get the basic idea. The scene in the book has Mielle (the performer in the video), at the end of the video lying broken on-stage in front of a screen from which a giant 'comedy' mask mocks her; as an image, it represents the moment where she in effect loses control of her public persona.
The scene stood out from a number of others which I considered for several reasons. Again, artistic ease was one of them - this sketch took hardly any time, and I don't think it will take too long to do a neat version. Another was that in addition to representing both content and theme of the novel, it has a resonance with the title; the mask comes across as a male icon abusing a female form - a textbook 'bad romance'. You have to study the novel very hard to figure it out, but to a certain extent the male lead character does abuse Mielle; it certainly wouldn't harm a reader's impression of the novel to have them associating Joe with the mocking mask here (ooo, spoiler? Maybe).
I'm currently having a devil of a time thinking of a cover for 'Heaven Can Wait'; I can't think of an iconic scene which would reduce in the way the 'Bad Romance' scene just described does, and there isn't an easy visual expression of the concept of a ghost trying to stay attached to life. If I come up with something, you'll be the first to know. In the meantime, I hope this has helped if you're struggling for a cover concept.