I spent most of the day working on cover art and went from
The first one appeals to me because the book is a history of the ice cream parlor (with lots of historically accurate recipes) and the heyday was approximately at the turn of the 19th century which was when the photo was taken.
But I'm not sure how well anyone can see it as a thumbnail.
The second one is bright and colorful and can be easily read in a small size, but it's a modern photo so I wonder if that misrepresents the book at all. The image is of a banana split and that is a recipe in the book so I'm fine with that part.
The great benefit to ebooks is you can change the cover endlessly and all it involves is an upload--that's something you could never afford to do with a hardcover or paperback.