To learn more tips about the importance of covers, and what distinguishes a great cover from a poor cover, download my free best practices ebook, The Secrets to Ebook Publishing Success. You might also enjoy watching the companion video on YouTube, where I speak at length about cover design and other important tips that will help your book be more successful. You’ll find the complete collection of Smashwords videos here: http://www.youtube.com/user/Smashwords/ Here’s a direct link to the video on best practices: http://www.youtube.com/watch?v=Jnd7jYcjqm8
Unlike print books which have a front cover, back cover and spine, ebooks only have a front cover.
Mechanical Requirements for Book Covers - Book covers must be shaped like a book cover (a vertical [upright] rectangle, not a square) and should include your book title and author name.
Keep your cover image tasteful and avoid obscenity.
1,600 pixels wide by 2,400 pixels tall is generally a good dimension, or such similar ratio. The image must be at least 1,400 pixels wide, and the height should be greater (taller) than the width. A good guideline is to make the height between 1.3 and 1.6 times greater than the width. I think 1.5 to 1.6 usually looks best, though you and your cover designer have some flexibility here.
You can view how different dimensions look at http://blog.smashwords.com/2012/06/new-ebook-cover-image-requirements.html
Book covers cannot contain nudity, a price, or a web address (Apple, for example, will reject your book if they see a hyperlink on the book cover image), or advertisements of items not already included in the book.