You can use an old method that Matt and Ben added several years ago. The idea is to create a hidden album where you would publish images to be used as custom thumbnails.
In Edit Album > Base Settings: Cover Image, when you click the drop-down, all the images in that custom thumbnail album will be available for cover images.
Here’s how. This is from Matt’s original forum post (from an older forum):
We’re going to publish a hidden “Custom Thumbnails” album. You will then be able to select custom thumbnails for your albums and album sets from the contents of this album.
Here’s how to get it done:
In Backlight, create a new album template named “Custom Thumbnails”, or something equally informative for your reference. The overall design or appearance of this album is unimportant. What matters, under Image Settings, make sure that:
Thumbnail Width & Height settings are sufficient for your desired album set template
With your album template made go to Backlight > Publisher, create a new album. (if using Lightroom, create the album there)
For Album, I use the name “_Custom Thumbnails”. The underscore keeps it at the top of my albums list, easy to find.
The most important thing, the Slug needs to be “custom-thumbnails” (without quotation marks); for Template, select the Custom Thumbnails template you made in step 1; and under the Features tab, enable the options “Hide from Album Set” and “Hide from Search”.
Add whatever images you want to use as custom thumbnails to this new album, then publish.
Edit any album or album set for which you would like to set a custom thumbnail. In the album settings, select the desired image from the Cover Image selector. You will be able to choose from any of the images in your new Custom Thumbnails album!
(Edit Album > Base Settings: Cover Image)
I’ve tested this and it still works in Backlight 4