Pictures grayed out in Photos card project

Ladies and Gentlemen: Request your help with a challenge with Photos. My wife is using Photos v1.5 on her 24-inch mid-2007 iMac. Using El Capitan (OS 10.11.6), which is latest version that this Mac can run.

She is trying to make a "thank-you card" project using a Photos template which will allow her to emplace 5 photos on the card. In the "All Photos" library the photos appear normally. However when she tries to pick the photos for the card from the project "photo picker" (my phrase), many of her photos appear grayed out and cannot be selected. You can see the image, but its faded and washed out. Other photos, taken at the same day, appear in full color and can be selected. (Double-clicking, right-clicking on the grayed-out photos have no effect.)

Again, the photos are in her library (they are not awaiting to be downloaded from iCloud photo library). We know this because (1) there are no cloud icons on the photos, and (2) even if we turn wifi off, the photos are still in her "All Photos" library and can be exported/modified.

What are the grayed-out photos trying to tell us? Why can't they be selected? And what can we do to use them?

Any help would be greatly appreciated. Thanks in advance // PEJ