When you add a background clip or an image—either a photo or a still image extracted from video—to your project, iMovie creates a clip with it that by default appears onscreen for four seconds when you play your movie. You can adjust the duration of this clip, making its appearance in your movie longer or shorter.
To adjust the duration of a photo, still frame, or background clip:
In the Project browser, double-click the clip whose duration you want to change.
In the inspector that opens, type the number of seconds you want the clip to last in the Duration field.
If you want this to be the default duration for all such clips added to your project, select “Applies to all stills.”
The setting applies to all images you’ve already added and to all images you add from now on. You can set the default duration only for still images and photos, not for background clips.