Hi,
This is something templates are not allowed to resolve. It is related to Virtuemart Custom Fields and how they work. We are hoping to see some improvements on Virtuemart3.0 and hopefully this issue will be resolved in the Virtuemart Core as well.
Basically, the "image type custom field" doesn't work based on child-products. So, once the product is added to cart, the parent picture is being shown. (instead of the thumbnail of image type custom field)
To tackle this problem, the "Generic Child Product" custom field should be used, therefore the child product will actually be added to cart, and its picture (which can be different from the parent product's) will be used in the shopping cart.
For the live example of "Generic Child Product" usage, visit this product:
demo.flexiblewebdesign.com/puremart/index.php/test-category/diesel-sweater-generic-child-variant-detail
(this example, is demonstrated one of our another template (PureMart) but the idea is the same SportMArt as well)
Regards,
Support.