How can I make sure a photo is shown just by itself, and not in a scene between other photos?

The trick here is to  use Custom Transitions, which you'll find either by clicking on the big red plus button or on the menu of each element in your timeline.

Place two transitions  before and after the photo you want to isolate, and you're done. Fade to black transitions are the best ones to separate parts of a video. The same trick can be used to force photos to be shown in small groups of your choosing.