Right, let's get macro..
So, you can assign a specific Catalog to an individual gallery image from Image Edit section of the account.
1) Go to Library
2) Click Galleries
3) Select a gallery
4) Hit the Edit (pencil) icon for a particular image.
5) You can now access edit the Catalog setting for this individual image.
6) Hit Save at the bottom of the page!
If you need a refresher on creating the specific Catalog (before running through the above process).. then lesson 10 in our Kick Start Guide will cover that for you.