It’s got to be a memory limit, do you mean 64Kb or 64MB? That’s still very low… it sounds like the layout (Grid/List/Default) within your WP e-Commerce theme is dynamically creating a thumbnail on each Page view which is why your memory is spilling over…
http://www.visser.com.au/shop/
That page uses thumbnail images saved to the server, on each pageview the same image is served up, no dynamic thumbnail generation is required. You might want to explore this option further.
