File size, dimensions (ratio) and resolution
File size, dimensions (ratio) and resolution are all very important when making your cover. To keep things simple, at the time of this writing, a good all around size for your eBook cover that will conform to almost all eBook distributors (Smashwords, Kindle Direct Publishing (Amazon), Kobo, Barnes and Noble, Apple iStore, and Bookbaby, Lulu being the exception) is 1600 x 2400 (6:9 ratio) at 72dpi. If you plan on using the same cover image for print, 300dpi is required. One thing to keep in mind are the delivery fees an author will pay when Amazon delivers your book. Authors are charged a fee for each book delivered according to the size of the eBook file, so the smaller the overall file size the more money in your pocket!
Creating your eBook cover images
When creating your own cover images, be sure to save your work in the native format of your program. I use GIMP and the native format is .xcf. Saving it as such retains all the layers and it will allow you to go back and edit the cover any time. I create my cover and add as many layers as I need. When I create a new cover I just add new layers to the file. Doing so results in a template with many layers but I can hide layers and unhide them at a later date if I decide to use them. I never delete layers in my master .xcf file unless they are ABSOLUTELY not needed. This way I can go back and create a whole new cover by hiding or un-hiding different layers. This also results in me having just a few master templates with many layers that can be used to create hundreds of different covers easily.
With that said, yes, you can have too many layers which will make the master file size very large. I recently created some premium Kindle Cover templates and had 20+ layers of just pictures. Not any pictures, full sized (2500 px high) pictures that took up a ton of memory. So much memory was used that I had to shut down a ton of processes and programs to get GIMP to continue to work. It took almost two full minutes just to save the file I was working on. That is a bit scary when you spend a ton of time creating a template like that and the program almost crashes because you have overloaded it.
I ended up dividing the template into 3 separate ones and they all work perfectly. Be conscious of how large the file is as you are adding layers to your master cover template.
Naming your eBook cover image files
I am not talking about the master files, I am talking about the completed cover files. I save mine as JPGs and give them a name that describes them. For instance, the file for the Kindle version of my book What I have learned writing and publishing eBooks is called writing-publishing-ebooks-kindle.jpg. Naming them like that helps me keep everything straight and easy to find.
If you want to create your own Kindle cover (they can be used for others as well as mentioned above) check out my GIMP templates. There are literally hundreds (if not thousands) of possibilities.
I can also make a cover for you if you need. Hey, some folks just want things done and I get it.
Do you have any tips about making eBook covers?