iMovie allows you to add transitions between photos and video clips. These are best used sparingly as too many transitions can distract the viewer from your content.
To add a transition:
Click on the transitions icon on the toolbar, then drag the desired transition between the clips. You can insert a transition wherever a green bar appears on the timeline.
Hover over the transition, click on the gear icon, and choose Transition Adjustments to lengthen or shorten the duration of the transition.
You can edit individual transitions or all of your transitions at the same time by selecting the 'Applies to all transitions' option in the Transition Adjustments window.