Kindle store customers judge the lack of cover art, or the presence of unaltered stock-photo cover art, to be a hallmark of the non-professional. You definitely want some nice cover art for your eBook. And it should match the cover art of any other editions.
Anything from a greeting card program to MS Paint can get the job done, but of course a dedicated graphics package such as GIMP or PhotoShop will generally offer more and better options.
In general you can use any public domain or purchased clip art in your cover image, but you may not use graphics copied off the internet unless their creator has explicitly given permission for commercial re-use of the image(s).
Size your cover art for 4"x6" at 300 dpi ( that 1200x1800 pixels ), it must be saved in JPEG or TIFF format with RGB ( not CMYK ) color mode set. This is different from the formatting requirements for image which appear inside the document.
Aside from stock photo collections, if you intend to use pictures where anyone’s face is recognizable, you must have a written and signed release from every living person who is identifiable or their estate if any of those people are celebrities. It’s best to stick with purchased clip art and stock photos.
Per title, the average publisher spends anywhere from $400 to $2000 for professional book cover design. That's a lot of money and they probably know what they are talking about.
So take some time, cruise the net. Try searching for 'Cover Art' or 'Book Cover Art How To' to find ideas and suggestions for your book cover.
Here's a YouTube video that shows how to make a 3D cover using GIMP free software:
The good folks over at BookCoverPro
have done a fantastic job of putting together a book cover editing package for around $100. They also have an eBook design special for $29
that might be worth considering.
Even if you decide to create your own the BCP web site has dozens of template examples that can provide a comprehensive layout concept.
Other interesting sites include http://ecoversoftwarepro.com/
for $27 ( on sale at the moment ).