Thank you for the response. My apologies for not including more detailed information in my initial post; it was my intent but I failed to do so.
The “Crop thumbnail to exact dimensions…” checkbox under Settings > Media is unchecked, which would allow for proportional thumbnails.
With regard to image settings within MarketPress, it utilizes the WP defaults or a custom setting. I’ve selected to use the WP defaults as shown in the following screen capture.
I determined the problem was with the theme because I created a new product, and uploaded a product image, to both of my websites (travelbooksusa.com and walmartatlas.com). The former uses the WP Jurist theme, the latter, Generate Press.
Before posting to this support forum, I spent several hours trying to isolate the problem and searched this forum for answers. I compared MarketPress settings for both of my websites to ensure they were identical. I compared WordPress Media settings in both sites to ensure they, too, were identical. I also used the “Regenerate Thumbnails” plugin on the walmartalas.com website (a suggested solution in another thread) to see if that would work (it converted all of the existing proportional thumbnails to what is seen now, the squared 150×150 thumbnails).
The issue isn’t a huge problem as it doesn’t prevent a visitor from ordering a book. It just doesn’t allow the visitor to see the entire cover, only a squared portion of a book’s cover.
Thank you for your time and effort in supporting this wonderful theme.