Assuming you have a PNG with transparency, and an iMovie project then, in iMovie:
- iMovie, Preferences
- Check “Show Advanced Tools”
- File, Project Properties, Timing
- Set Initial Photo Placement to “Crop”
- Drag the PNG with transparency to the point you want the overlay to appear in the timeline
- From the resulting pop-up menu choose “Overlay” (for full screen) or “Picture in Picture” (for part-of-screen)
- Adjust the size and duration of your overlay