This looks fine, but there is many things why can't find that image. I will suggest you, instead of attachments to use media selector field (add custom field on product, f.e. ProductImage), and also mark new field as 'searchable' and 'content'. After that, you can use in your transfomation:
<%# Eval("ProductImage") %>
This will retrive you url that is placed in ProductImage field.
Hope this will help.