I was a bit confused over the image height ratio but when I tried 140 the book cover graphic fitted close to perfectly. It's a bit trial and error to work out the scaling but I guess if you did some calculations from the original you would get it right first time.
I tried the Image Widget plugin and it was a lot more fiddly..in fact I couldn't get the image to scale correctly. Another option is included in the WP Jetpack, but haven't checked that for scaling yet.
Responsive Image Widget is highly recommended from what I can see so far.