Vinson Checkout for SiteBuilder - Using custom image field for matrix child item in checkout summary

What is the best way of getting the secure URL for a custom item image field for use on the cart summary in the checkout for Vinson for SiteBuilder?

I'm using Vinson for a site that uses mainly matrix items, and each child item has such a unique look, it's best to display a child image in the cart and checkout. I've already made the item detail template display the correct matrix option image from a custom item field with the image set on a custom item field the child record.

I've changed the ItemsKeyMapping to grab that custom image when an item is a matrix child item (98% of the items on this site). This works great on the shopping cart. It is so less confusing to the customer when they add a green recycling bin with a special top to the cart and actually see the exact child item they ordered. However, what NetSuite returns for the custom image field in the itemsKeyMapping is the file name, like