Here's what I'd been told: Never buy the extra-small size picture. Always go for extra-large. But although I was new to the cover-design end of writing, I figured I knew best and bought the smallest (eg cheapest) images for my first book cover.
Here's the dilemma: you can easily make a large picture smaller, but you can't make a small picture larger without sacrificing the quality of the image. See, small pictures will pixelate when you stretch them out. For me, this meant that while my cover image looked wonderful as a 480 x 480 px size, it blurred into nothing when I attempted to increase it.
Sure, the larger sizes cost a little more, but trust me, they're worth it. After my first fiasco, I purchased another set of images, larger ones this time, to make my book cover. Since everyone judges a book by its cover, having a terrific image is a necessity, not a luxury.
Photo credits: Willeecole.
All about color