If you play the footage in the timeline now it will now play your timelapse at 10 frames per second. Photos and videos often have different sizes which you can fix by cropping the photos to the right size. You can adjust the crop so that all the photos fill the frame properly.Use Command + A again to select all your photos and drag them into the timeline.
This means that iMovie will see every photo as a video clip that is 0,1 seconds long.
