Add attribute to thumbnail image in Custom Fields

[CMYKreative]

OK, we've got MagicZoom installed and working on our product images, but I also need to add this to a custom field on the right hand side that shows the material swatches:

http://demo.cmykreative.com/berkeley/index.php?option=com_virtuemart&view=productdetails&virtuemart_product_id=38&virtuemart_category_id=11

You'll see that we've added an "image" custom field to the right side where the user chooses the material they want. I need to add some additional code to each of those thumbnails to show the MagicZoom enlarged image, so for each thumbnail that appears there, I need to add:

rel="zoom-width: 400"

So, how can I do this? I'm thinking that in the "product" (where we add the actual images) there should be an additional 'field', but am not sure how to go about it.

Help appreciated!