Site specs: PHP 7.0 / WP 4.7.2 / custom Gavick Quark theme / MarketPress set to default css
Support access provided.
Under the Settings > Presentation menu
Show Product Image: Yes
Images size: Custom
custom size: 500 x 500
eg. Product page http://cherryblack.com.au/wp/store/products/the-lost-mayan-blend/
however blank placeholder displays and inspect shows that it is loading http://cherryblack.com.au/wp/wp-content/uploads/cbr_250g_w-1x1.jpg
Additional note: Marketpress ajax cart js doesn't work under PHP 7.1