bullfrog, having changed the maximimum image sizes, have you regenerated the image cache in tools > maintenance?
In skin/reboot/customer/main/product_style_1.tpl
Try replacing...
Code:
{include file="product_thumbnail.tpl" productid=$product.image_id image_x=$product.image_x image_y=$product.image_y product=$product.product tmbn_url=$product.image_url id="product_thumbnail" type=$product.image_type noborder=true}
With...
Code:
{include file="product_thumbnail.tpl" productid=$product.image_id product=$product.product tmbn_url=$product.image_url id="product_thumbnail" type=$product.image_type noborder=true}