I found a plugin in your forum through the wpmu.org blog link.
Product grid shortcode for MarketPress [mp_product_grid]
Version 0.2 | By Nick Daugherty (Skyrocket Websites)
I dloaded/installed (mp-grid-layout-plugin.zip) and everything seemed dandy till I tried to view the store's product via grid view shortcode. [mp_product_grid]
The actual product image is not displayed but the title, add to cart and price is displayed. I'm racking my brain in figuring out why and since there's no settings, I don't know exactly where to look or configure to make it display properly or what I'm missing.