Stills keep snapping to wrong place on timeline
Just when I think I have iMovie figured out, something happens. I have this long piece of narration that I've imported into a new project. Now I want to add stills to illustrate the narration along appropriate points of the timeline. I have added three stills so far but when I try to drag them to a different place along the narration in the timeline, they keep snapping back to the beginning as shown in the screenshot. This has never happened to me before, but then, I don't normally start with a long piece of narration. How do I fix this?
Mac mini, macOS 10.15