Problems using snippets

Replacing images in all snippets no longer works. A new image is always inserted in the storyboard

Snippets don’t seem to be working at all. I dragged and dropped two images on to one that I sometimes use: the one called “2 vertical”. Was left with a blank screen.
I then tried one from Pack 1: “3 sliding left”. Again it didn’t produce a result. And this case, I wasn’t able to delete the snippet and am left with a blank black screen between two other slides.

The blank screen was ticked as “Locked” for some reason. When I unlocked it, I WAS able to delete.