What will the user do with this list of thumbnails once they've selected the categories?
If you create this self join relationship:
Product Catalog::gCategories = Product Catalog by Category::Category
a Portal to Product Catalog by Category will list all the records that have a category matching one of the categories selected in the checkbox group formatted gCategories field.
gCategories would be a text field that you would likely want to specify global storage. (That's why I added g to the begiining of the file nmae)
You create "Product catalog By Category" by selecting Product Category in Manage | Database | Relationships, clicking the button with two green plus signs, then double click Product Category 2 to bring up a dialog where you can rename it.