During 2019 this feature broke and instead of seeing a mosaic thumbnail for Flickr Collections, we got 12 gray boxes. Flickr commented on the issue but kept us waiting:
"Collections: Mosaic cover images in Collections are currently unavailable." - Flickr
Now they are back, but in an unfortunate way. The Flickr API still can't serve the mosaic as a single image like it used to. Only via an authenticated call (adds complexity to setting up Flickr in JIG) would it be possible to get the list of images that make up your mosaic, but they would still need to be stitched together. Else, JIG would just pick one of the photos in the mosaic. That's only slightly better than what we have now.
Starting with JIG v4, the grid will pick the cover from a random album found in the collection you are showing, if that collection has any albums. Still, the collections in a collection scenario will result in box with 12 squares.
Here is how you can change the cover of an album. It's not necessarily the first photo in it.
That's the best we could do that is performant and doesn't require changes to how you use the Flickr feature.