The entire image on the cover would be best. The size of the cover is 6”x9”
I know the pain of having to choose between a part of the image you liked for the cover just so it fits proportion, and wanted to avoid that. When I tried finding a similar image on the net I found a better resolution one that scales in proportion to 6"X8". I just dragged the image horizontally 1" to suit the desired proportion hoping the now distorted horizontally oblong image does not appear skewed. I then added a 1 inch trim on all sides as shown here.
Your full image (300 PPI) is here:
Original image that scales in proportion to 6"X8" (72 PPI) is here:
You have an alternative too - an image that is similar appears on Getty Images and its number is
1176551041 if you want to purchase it at full resolution.
Note: If your book is Portrait, as in 9" is the height and not width, then of course all the above images will not help.
All the best with your book.