Another helpful link thanks to Barnes & Noble's PubIt page on Facebook took me to this article: 3 Secrets to e-Book Cover Design Success. Since I've been thinking about self-publishing, and since I've already done some work designing a cover, I was interested to see what the article had to say.
Basically, according to this article, your ebook cover has to be simple, small, and branded — simple and small because customers have to be able to see and read the covers in those little thumbnails that show up in search results on Amazon, BN.com, etc., and branded because catchy covers sell better than those that are boring or don't have a theme.
Looking at my covers, I definitely have a recognizable brand, since I'm using 1920s postcards for all of my covers. Customers will be able to recognize one of my books in the series right off the bat, even though the cover illustration will be different. Even better, the brand I'm establishing here won't be confused with anyone else's covers because, to my knowledge, no one else is using old postcard images for their covers.
But do they shrink down well enough? I'm not sure. Here, I'll let you be the judge of that:
I think this size should be close to the thumbnails shown on Amazon and BN.com. What do you think? I know the "And Pandora's Box" is harder to read than the rest, but I feel like that part isn't as important. "Ruby Ransome" is the part I want to stand out, and with a long title, I don't think it's realistic to want it all to be easy to read.