I created a simple cover image for an e-book at the end of last year, and found that the online references are few and far between when it comes to defining image sizes. The best one is by Smashwords – themselves an e-publisher – and cover not only examples but an explanation for the layman of topics such as image proportions.
Many of the online references I found are out of date because of the wave of higher resolution screens which have become available over the last couple of years, like Apple’s Retina devices. Because the resolution of these devices is much higher than older devices, the cover images need to be at least twice as big. E-book readers seem to shrink over-sized images automatically to fit their screen size.
The general consensus seems to be to find the image proportions required by your publisher (e.g. 1:1.6 for Kindle) and then to create a JPG in three-colour RGB format between 1500px and 2000px wide. For Kindle, the recommendation is 1563px x 2500px.
A maximum file size of 2 Mb applies at most publishers, but optimize your finalized JPG (e.g. using JPEGmini) so that the produced e-book takes up as little space on the reader’s device as possible.