Here's a collection of book covers I've designed covering a wide range of subjects, both fiction and non-fiction. I usually produce four different designs for a client to choose from. (The cover design selected might not be the one I prefer). The artwork is of course produced at a much larger size than this, generally over 1000 pixels deep although as they generally appear at a small size on online bookshelves I think it is important that they look strong at small sizes too. All images used are either hand-illustrated or constructed from purchased images from photo libraries. All are royalty-free.
Printed books have to be produced at a higher resolution, 300dpi, and also include back cover, spine and barcode. These incur more work as a one-piece PDF cover has to be created. The thickness of the spine will vary depending on paper used and number of pages within the book. A spine width calculator for Createspace POD books can be found here.