Low resolution (72 ppi) graphics are suitable for the web. If you’re interested in using the graphic for a web site (banners, backgrounds, etc.) or you will be reducing it in size, then the low resolution version might work well. If you will be enlarging part of the image, however, consider getting the high resolution version.
What about e-book covers? Here you have to think ahead. If you’re sure that you’ll only release this cover for e-books, then low resolution graphics may be suitable. Reasons for not going with the low resolution, even for e-books:
- If you decide later to release in print, you’ll have to re-create the cover in higher resolution (300 ppi). Using Photoshop to increase the resolution may not give you the results you want (there are products that have more sophisticated algorithms for increasing size and resolution than PS, such as Alien Skin’s Blow Up, which will work better).
- Again, determine whether you’ll enlarge the graphic. For instance, if the character face you really want is in a model shot shown head to foot, you can end up with pixelation and blurring.
What about covers that will (or may) be for print books? Always work in 300 ppi and get high resolution graphics for your composition.