There are a couple of options for inserting still pictures into your video.
In iMovie you can upload a picture to your timeline in the same way as you would upload a video clip. The default setting is to display the picture for 4 seconds but if you click and drag the yellow border you can adjust the time.
In other versions of video editing software you may need to paste the same picture into the timeline multiple times to get it to display for longer than one frame.
Your graphics can be photographs, presentation slides (in powerpoint use “save as” to change the file type to a picture format), cartoons, scanned hand-drawn images, or a freeze frame from the video (in iMovie, “Modify > Add Freeze Frame”).