As everyone probably knows by this time, I have a very dim view of stock art. Its use has become something of a crutch. I have seen too many people settle for second-best simply because they cannot find the right artwork available on line. And they stop looking right there, as if there were no other alternatives, no place else to turn to.
There are a lot of problems and issues with using stock images, most of which I have already discussed elsewhere. But to sum up---
Too often the author/designer has to settle for "close enough" if they cannot find an appropriate image.
There is the danger of having your cover art appearing on a hundred other books at the same time.
There is also often the cost to take into consideration.
And, finally, it is too easy to become dependent on stock imagery. One of the results of this dependency is a stifling of the imagination. Far, far too many times I have seen someone give up on an idea entirely because they can't find what they want ready-made online.
This is especially frustrating to see when the author is looking for something that is not even especially unusual.
All of this is made even worse when when the image desired is something that is, in fact, immensely simple and perhaps even commonplace.
One solution to this dilemma is: Why not create your own "stock" art? If you have a phone you have a camera perfectly capable of taking images of high-enough quality to use for a book cover. There have been countless examples in these forums where I have seen someone have to either settle for second-best or, worse, abandon a good idea simply because they couldn't find the right image ready-made. This is especially frustrating to see when the necessary image would be something immensely simple and easy to create.
What stock imagery should be is a last resort rather than the first thing one thinks of. It goes without saying (but I will anyway) that not everyone might have what's needed for their cover. There may be props, costumes, settings, etc. that may be difficult or impossible to find at hand. The point is, eliminate the possibility of creating your own images either whole or in part before turning to stock art sources.
A cover may be a mix of found images and created ones. A background, for instance, may be from a stock source while the characters in the foreground are your own models.
Here are some examples of the sort of thing I would like to encourage people to do...
The cover for Lois Bujold's Knife Children is a good example of what I am talking about. http://black-cat-studios.com/webtests/1000_(6).jpg All I needed was two hands making the right gesture. It was simplicity itself to take my own photos rather than search all over the internet looking for something that might only be just close enough. (The background was composed of images I already had in my files. The knife was yet another file photo, hand-altered to match the book's description.) The same also goes for Ars Poetica, where all I needed was a poem printed on a sheet of paper and a glass of what I hoped might look like liquor (it was tea: I'm not going to spill good liquor for a book cover). Again, a few minutes' effort and I got exactly what I needed, without any need for compromise or half-measures.
I have included a dozen examples of cover art in which I have employed my own photos, along with a few examples of the original photographs. The only "stock image" source I will turn to is the Library of Congress, which is a gold mine of background details. For instance, the interior of the room in the Lovecraft artwork, or the ruins in the distance in the space pirate illustration.
Some of these examples may still be a little more complex to create than some people may be willing or able to accomplish, but the underlying point remains the same: don't automatically turn to stock image sources, don't develop a dependency on them and, finally, exercise your imagination!
I also take photos wherever I go of anything I think might be even remotely useful later: trees, landscapes, rocks, mountains, buildings, cars, animals, textures of all kinds, props, machinery, etc. etc. I keep these in files sorted by subject. The background for Exchange of Hostages, for instance, was composed of several photos I took of machinery and construction sites, while the backgrounds for other images were found among photos I had taken during various vacations, such as the rocky sea shore that is in one of the examples, or a room in an old castle.
A great source for potential models, props and costumes is a local Little Theatre---or a high school or college theater department. Organizations like these are often happy to help out a local author. We have a very active Little Theatre in the town in which I live. I have not only found willing models from among its members, but its collection of costumes and props has been invaluable. There is scarcely any period of history for which I can't find what I might need. Fortunately, I can also depend upon my immediate friends and family (many of whom show up in the samples I've linked to).
The main trick in composing a cover from several different images is being aware of light and color. All of the elements need to look as though they are part of the same scene. For instance, in this picture http://black-cat-studios.com/webtests/1000_(9).jpg I made sure that yellow light from the background spilled onto the figure. In this example http://black-cat-studios.com/webtests/the_magi.jpg , even though my wife was photographed someplace entirely different than the castle chamber background, I made her part of the scene by making her drapery translucent so that light from the window showed through it.
One useful technique that goes toward eliminating any cut-and-paste look is to run the Blur tool around the edge of a picture element. This removes the hard edge and makes the image blend ever so slightly with the background.