I've been interested in graphics ever since I first got a PC. That was back when MSN still had their groups and I joined one that taught Paintshop Pro and also HTML. Yeah, I'm old.
I stumbled into professional cover design by the simple process of making mock covers for my own work – being able to see the “finished” product was a huge incentive for me. When I signed ELEANOR'S HEART over to Champagne Books, I had one such cover and enquired as to whether I could use it. Not only did boss Ellen Smith say 'yes', but she asked it I wanted to create more covers.
It seems that a lot of self-published authors don't think about covers, which strikes me as odd. What is the first anyone is going to see about your book? Cover. What's the best way of advertising it? Cover. And yet some people scrimp on this vital part.
Creating a decent cover requires a graphics program which lets you manipulate pictures, a vision of what you want, and an understanding of typography. The cover has to capture a snapshot of the book – indicating genre and theme. So a sci fi romance ought to have something spacey (technical description) and a couple. Avoid clichés and cluttering, and keep it simple.
Remember that most purchase sites will reduce your cover to a thumbnail – check how it looks as a reduced size and ensure it's still legible.
It you're buying stock images there are two important things to remember. First, check the license and ensure it's one that covers everything you want – don't forget that you're not only going to be using it for the cover, but for all the promotional material. Secondly, the minimum resolution you want is 300px, even if you're just publishing electronically. Again, when it comes to physical promotion – such as bookmarks – you want quality to avoid “fuzzing”.