You can accomplish this with themes.
First, you need to make sure you have themes enabled.
Make your collection of royalty free images into a public collection and give it a theme name.
Then, use the "j" permissions in group setting to allow your groups to see the theme category. Be sure to also set the "J" permission, limiting search results to resources within accessible themes.
Your other option is to use the group's search filter area to force results to include specific data within a given field. This method would be nice if all your royalty free images were tagged as such. You can find specifics on how to do this from the wiki (though it seems to be down right now). There should also be enough information within other group postings to get you through.