I want to superimpose my logo on a video in iMovie- how do I do this?

You can use the Picture in Picture feature to overlay the logo. To do that put the logo into the timeline that is above the video clip. Then select the logo by clicking on it. After selecting it, click on the Overlay icon (the one that looks like overlapping squares) in the tool bar at the upper right of your screen. In the selection box that appears, choose Picture in Picture from the drop down menu. Your logo will appear on your screen overlapped into the video. Adjust its size and position by dragging. Then click on the blue reset button above the screen and to the right to apply your settings.

-- Rich