To set up a backdrop as a recommended backdrop, go to Manage --> Backdrops and click on the backdrop you want to edit.
In the right sidebar "Visible to Packages", select the packages that you want this backdrop to be visible to (ie. can be booked with).
Make sure the package is checked/visible.
Click the star to the right of the package to make the backdrop you are editing recommended for that package.
When a client books one of the packages with a recommended backdrop, the recommended backs(s) will show up at the top, above the other backdrops.