Adding photos to your movie and using "snap to beats"
From Help Wiki
Using snap to beats in Imovie is great if you want to put a photo collection to music. Snap to beats is automatically on when you open Imovie. Drag photos directly onto the project. This feature is perfect to make a photo montage in the middle of a larger piece or by itself. You can control where each photo will apear and have complete artistic freedom. You can even turn off snap to beats if you are feeling antonyms.