So, here are a few things I’ve figured out that are probably applicable to whatever you’re doing. Two things which bear repeating (like the mantra that to be a writer, one must read and write) are:
1) The title must be readable.
2) The thumbnail must look nice.
Ultimately, while you’re trying to entice a reader to pick up your book, you’re also trying to convey useful information, hence you want your title to be easily readable. If you’re having the problem where you’re putting a title over a busy background, you can take the path tool, draw a box around the text (hit ctrl to close the box), copy the path to the selection, and using a new transparent layer, dump a color into the box with the bucket tool. If you want the background to show through, you can make that layer transparent (in the layers dock, right near the top you’ll find a slider for that). Please note: if you anchor down the layer, you will lose the transparency. You need to merge the layers instead.
Here’s some more general advice.
Do it again
It’s fine to use your first draft of a cover if you just want to get the damn thing out the door (I understand that), but you’ll probably want to design at least a couple of different looks. And when you’re first learning how to deal with hair, masks, etc. it’s okay to start from scratch if you get frustrated. At this point, you probably need the practice.
When I find my books are doing nothing, like absolutely nothing, I go a redesign my covers. I love the garden look I have for Love on the 500, but it’s not moving books so it has to go. I’m getting ready to publish Gutter Punk soon, the second book, and I’ll probably try and redesign Love on the 500 to work with that cover. I’d be lying if I said I wasn’t bummed out though, because I worked hard on that cover.
Don’t be afraid
Ctrl+z as many times as you need. Look up five different tutorials until you get it. I’ve found that fooling around with stuff, I’ve figured out a lot. Using the curves on the path tool can be a little clunky at first, but you’ll get the hang of it. Take a break too if you need it, because it can be frustrating.
The eyedropper is a great tool
In order to keep a cohesive feel, I often use the eyedropper tool. Like, for bokeh dots, I’ll use the highlight color of the woman’s hair. For the garden cover of Love in the 500, I used the same hue of blue and red throughout, and if you write down that number (it will be among the first sliders available) you can make it lighter and darker as needed. Using the same colors will help tie the whole image together. For my Tale of Two Clitties cover, I colorized the background of the bottom image to match the blue from the top image. With the right hue, I just eyeballed the other attributes (saturation and vibrancy).
Creating the feeling of dimension
Using darker hues, smaller sizes, and sliding around the layers so things overlap can create the illusion of dimension.
You can do so much with the copy feature
You can copy things into new layers, new images, as brushes, whatever you want! I’ve had a lot of fun with this feature. I wanted to create a kind of shadow effect with the title of this one story, so I made a brush out of the title, set the transparency to about 50% so I could get this kind of after image effect. I don’t know if it worked that well, but it was fun.
If GIMP gets wonky…
Maybe uninstall some of those brushes. It could just be the version I have (I’m not sure when it was updated) but if I install all of Obsidian Dawn’s brushes, it gets super freaking clunky.
You can do a lot with masks
If you’re working on hair or something, remember coloring something black on the mask will make it transparent, and coloring it white will make it opaque. I use masks for lots of things. When I make a box for text, I like to have texture on it. With a mask, I can make sure I’m just coloring on the box and not all around it. For text, if you want to have a kind of beat up look, you can make it look distressed by adding a quick mask and putting a texture on it.
Some of the things I’m talking about are at an intermediate level as opposed to basic. Understanding them will allow you to do a lot more with your covers. There is an element of trial and error to this. I’m not the best at making covers, but it is something I enjoy. If you don’t enjoy it, you’re probably better off hiring a cover artist (advice for that here). If you don’t have the money, make a basic cover, because it’s better than nothing.