A lot of self-publishing authors are having a go at designing their own book covers. Yes, I know people say that you should have it done professionally, and yatida, but if you have a lot of shorter works, that quickly becomes very expensive. At 99c per short story, you have to sell a lot of stories to recoup the cost.
If you’re reasonably handy with graphic programs, you might try to make something yourself.
So what should a cover do?
It’s got to be attractive, it’s got to represent the genre, and it’s got to look at least semi-professional. I want to say a word or two about that last item.
As to representing the genre, have a look at covers of other books in the genre. Decide what images to use. Ask random people what genre they would associate with your proposed design. A lot of this, including attractiveness, comes with taste. This is why getting as many opinions as possible is a good idea.
In addition to this, there are a number of easy things you can do to make your cover look better:
Colour scheme: define a colour scheme for your cover with a dominant colour (e.g. sky blue) and make all or most other colours shades of this hue. This means all mixtures of your dominant colour with black or white (and this includes grey). You can use one (but not more) discordant colour in a very small proportion of the image. Consider the tone you want to convey. Given the above sky-bue theme, what emotions would you associate if the discordant colour is red, yellow or green?
Alternatively, if your cover is very colourful, and doesn’t lend itself to a colour scheme, make sure the hue and saturation of the various components of the image match. You see home-made covers where an obviously cut-out and pasted part of the image is much darker than the rest, or has much more contrast, or has the wrong hue (for example, it’s reddish where the rest of the image is blue-ish).
Lighting: related to the above, if you are copying an object, say, an image of a man, on top of another image, say, a landscape, make sure the direction and strength of light source matches between the two image components. A picture of a man taken in sunlight superimposed on a rainy/misty landscape is going to look fake and horrible. You have to fiddle with the contrast and saturation slides until the tone matches.
Cut-outs: use the most appropriate methods for removing desired sections from component images. If you’re cutting out an object that has a good number of smooth lines and curves, for crying out loud, lean how to use the path feature. Not only will it give you a smoother selection, but paths are vector-based, so they will remain smooth, even if you enlarge them. Even if you use magic wand or other pixel-based methods for selecting cut-outs (and there are good reasons for using them), make sure you use anti-aliasing, smooth edges and if necessary the feather features to make the result look less like a–well–cut-out.
Text: resist using the discordant colour as a flat fill for the text, unless your font is huge, bold and simple. This works really well for crime novels. But, if your text is going to be bright red or purple or yellow, or any colour that’s painful, keep the font really, really simple. If you can, try to avoid fonts that everyone is using. Yes, some of them are pretty, but they get old pretty quickly.
Alignment: make sure the text is properly aligned, and uses the full width of the page. If there is extra space, enlarge the text until it fills the width of space you want to use. You can either center your text or align it along either margin, but if you center one line, but keep another line right-aligned without any obvious reason (such as features in the image you don’t want to cover), it’s going to look untidy.
None of the above will guarantee that someone with a design degree won’t pick your image as home-made, but it will result in a tidier, neater cover.