If you make PNGs the placement that you reference as annoying may be resolved. For example, pillows try to fit the box when a jpg but when png, you can pick background color and adjust the size - it is ah-mazingly better.
Also, like many others mentioned, it is a combination of marketing to send people to your site (be social media savvy to do this for free) AND a numbers game (get more pieces, I started seeing regular sales sometime after I had well over 150 pieces).