For many DIY authors who have either a familiarity with software and aspects of design, access to great templates for covers, or just want to try their own hand making covers, knowing where to get license free artwork is a boon.
One quick caveat on cover art, many people have made many, many really bad book covers. Good enough to satisfy doesn’t cut it. This is one more area where the failure of indies (or their apathy towards the subject) has made the term “self-published” synonymous with “crap.” Please don’t take a dump on the pile and add to it. If you are committed to releasing a DIY cover into the wild, please make sure that it doesn’t reflect poorly upon indies as a whole. There are many services who will gladly contract with you to design the cover you have in mind (but check their portfolio, first, to make sure he or she isn’t some fly-by night with a demo copy of Photoshop striking out with no more skill than you.) If you see multiple images that you’d love merged into your ideal graphic but don’t have the skill to seamlessly integrate them, please don’t go alone.
There are many places to acquire stock images for use in your covers. http://www.istockphoto.com and http://www.shutterstock.com are both highly recommended. I have memberships at both and use Shutterstock often. I have also been a member of http://www.deviantart.com for many years. Its forum has been a mostly fruitful place for me to hire professional illustrators for a variety of writing-related commissions.
For the ultimate in DIY on the cheap you can search for free images at sources that aggregate CC0 license stock images. Please use these places and not Google Image Search. A Google search will show copyrighted images and you can quickly find yourself in legal trouble for infringement—and an author who willfully infringes on the copyright of someone else (pretty close to plagiarism) will find little support from the community at large. CC0 stands for Creative Commons Zero. Under the terms of the license, all images may be used, displayed, or modified freely for personal or commercial use. The only stipulation is that identifiable faces should not be used in potentially offensive ways (like erotica book covers). No attribution is needed. (More here: https://creativecommons.org/publicdomain/zero/1.0/)
My favorite site tops this list, others follow.
Picking your cover art and design motifs are one thing, but remember the practical things, too. Your spine’s thickness will depend upon exactly how many pages are in your book, and whether your pages are white or crème (they have different thicknesses that add up.) Rather than do all of the math, I’d recommend searching online for a “book cover calculator” that will adjust your spine width for you and give you a template to design over with proper bleed and trim edges. Both Createspace and Ingramspark have these features built into their creation wizards and allow users to upload their own files for quality proofing by their own experts (mostly Skynet computers who gained sentience.)
Because of the large file sizes and opportunities to constantly tweak, change, or mess up, I recommend saving often, and save in multiple formats. I always save any final WIP that I intend to upload as a Photoshop PDF with all layers merged together and at 300DPI (always work in 300DPI—you can always lose quality, but never gain it). Some publishers have maximum file sizes to keep their servers from imploding; merging the layers helps bring that size down.
When all is said and done and you are done tweaking and adjusting your graphics, make sure to get a physical proof ordered by your printer. Oftentimes colors will represent as darker or lighter when ink hits the paper. I try to intentionally upgrade the vibrancy of my covers so that they pop more. Printed covers tend to either look darker or more faded out than they do on the screen.
Happy designing, and don’t feel bad if you get unhappy results. I’ve scrapped many pieces that I didn’t love… and my biggest regrets come from using artwork that wasn’t quite ready for the world to see. Don’t publish unfinished art or unedited stories. The world won’t thank you for not doing that… but it will ridicule you if you do.