Contrary to popular belief, IOHO the art of making a good Kindle Cover [Product Image] is an Art of Marketing and not an Art of Graphic Design.
That is because we are not talking of an Art Competition but a means to SELL YOUR BOOK.
So the cover should simply do its job to GET the putative buyer [surfer] to download a Sample of your book and thereafter the book must stand on its own two feet [which then morphs to the FORMAT question of how well did you do THAT task].
A surfer generally first meets your book [from a search, in this case "stranger"] as a tiny thumbnail in a column of thumbs, as seen here for Strangers on a Bus:
The thumb is 115 x 115 pixels so the first piece of logic says use ALL of that. As seen, the other two thumbs use only 66% of the space. Secondly we are of the belief that "Roadkill" is OK for the INSIDE cover but 3-D "pops" far better, looking like an actual book.
Next obvious logic says make sure the title can be read at this size as most people will be scanning [very quickly] down the column of images and not the description on the right. You have only several milli-seconds to grab them as they flash by.
So IF you DID stop the surfer halfway down the wave and they clicked on your thumb, the next task is to KEEP his/her attention, ie to click the SAMPLE button and not the BACK button to go back to the list of thumbs.
On your side is the fact that Amazon has grown your thumb to 300 x 300 meaning the surfer can get a better look at your cover as seen below:
However you need to be aware that Amazon lobs a Kindle into the bottom-right corner, so for author's name or the red icon in this case, best to keep it to the left if you want people to read it at this size. Remember that the surfer always has mouse poised on the dreaded BACK button so YOU have to make it EASY for surfer to like your offering.
If the surfer retains enough interest to click the 300 x 300 image it then goes to 500 x 500 [and without the Kindle icon] which is as big as it gets [but see under re Zoom], but as we said from start this is NOT an Art Exhibition and surfer is far more interested in the question "What is the book about?".
As seen later the description requires the surfer to scroll down the page, and that is asking a lot if your Attention Grabbing at above TASKS was lacking, meaning there is far more chance he/she will press BACK and you have to wait for the next surfer [who will probably do exactly the same].
But here is another bit of Amazon genius that you can use. In the image above you will see that we provide a FREE rear cover with "mini description" to our Customers as a Customer Image, and if the surfer just passes the mouse over that image he/she gets this:
So here is the answer to the surfer's question WITHOUT needing to scroll down, and if it is hard to read, a simple mouse click gives the full 500 x 500 size, per:
But let's see how a cover selected at random [this was sitting under at "last viewed"] measures up.
The law of averages says it WILL be roadkill and it is, per:
Firstly the author name is partly covered up by the Kindle, but the Double-Whammy here is if image is roadkill, the Kindle is moved further to the left, thus obscuring more of the already undersized image.
Secondly Amazon is trying to forget about their Zoom feature BECAUSE it distracts surfers from going to the Sample, so they say MAX size of upload of 1280 pixels, and if you obey [we upload 900 x 900] then no Zoom.
But thirdly, and most importantly, there is no rear cover description so the surfer is asked to scroll down the page, which few will do with 100 more thumbs to be examined from the search.
If we look at the description down the page it seems like a very interesting book BUT the question is how many surfers go out of their way to SEE it, per:
Finally we can backtrack to the thumb per:
At thumb size the title can only just be read, and for sure does not "pop" at all to grab the surfer passing by at 5 to 10 tps [Thumbs per Second].
So our final comparison [on a Lester Bangs take it or leave it basis] of these two essentially random selections is that the latter with its sepia tones and eerie feeling would win an Art Competition hands down over our Greyhound Bus "commercial" rendition, BUT the Bus one would sell more books.