Thumbnails for buying online

More and more people are buying books either online or as ebooks, which means what people see when they're browsing are tiny thumbnails. So what makes a thumbnail cover, as opposed to a print cover, stand out?

Of the above, the ones that stand out are the ones where the design is:

• bold with lots of contrast
• uncluttered
• where text is an important part of the design, it is large and in an easy-to-read typeface

The Steve Jobs biography stands out the most for me, even though the type is impossible to read at this small size. This doesn't matter though, because the title is displayed immediately below the cover image. The black and white also translates well onto e-readers that use only e-ink and don't display colour.