Hi Timelikeit!
Here is the way I do it, in Qicktime Pro - much easier than trying to use plugins that have not been updated for Tiger and often don't work:
Are you sitting comfortably? Then I'll begin!
Organise the video clips and/or stills that you want to superimpose on to your main video clip, by having these in separate, individual iMovie projects (no matter how small). It helps if these are in the exact length/time you want later.
Now start with the clip in your iMovie project on to which you want to superimpose one of more smaller video clips or stills. Lets call it Main Clip. Export this (share) to Quicktime Pro in DV (full quality). make sure it has the same aspect as before (Quicktime tends to default to 4:3. If you want 16:9, read on).
In Quicktime, with this project open, select File ~ Open File. Select the iMovie file (in your movies folder) of the first clip/still you want superimposed. This will open in its own QT window. Select Edit ~ copy. If it is a movie clip then select all ~ copy (or you will only get one frame!). Switch back to the Main Clip window, and click in the scroll bar where you want the extra clip/still to appear. Choose Edit ~ Add to Movie.
You can repeat this as many times as you like, if you want to build a 'Video Wall', i.e. have several superimposed clips/stills on the same Main Clip.
Do not worry that the new clip overlaps or covers up the Main Clip, at this stage.
Choose Window ~ Show Movie Properties. The Properties Dialogue appears.
You will see a list of video (and audio) tracks. Track one will be the Main Clip, track 2 your new added (superimposed) clip. Click track 2, then click visual settings. From the Scaled Size pop-op menu, choose percent. Then type 50 into the first box. If Preserve Aspect Ratio is turned on you won't need to type 50 into the second box. You have made the pasted footage appear at one quarter its original size. The inset is now hugging the top-left corner of the Main Clip. If you want, you can type numbers into the Offset Boxes to shove it away from that corner (there are 72 pixels to the inch).
If your original Main Clip was required in 16:9, unclick (de-select) Preserve Aspect Ratio, and type in 1920 in the first box, and 1080 in the second, in Video Track 1. This will not affect video track 2, but if that is also wanred in 16:9 repeat for that track.
You can superimpose, 3, 4, or more such 'overlaps' on the same clip, and spend a happy afternoon tapping away for each track in the Offset Boxes until you have got them all in the right place! You can even overlap them, if you can remember (or note down - hire a secretary to take notes) which track you want on top etc.
Save the completed QT file. Import the completed file into a new iMovie project. Import this into the original iMovie project, substituting the old clip you copied to QT as Main Clip for the new fancy clip you have made.
You will probably need to experiment a bit with the above. I know, it took me two days to master it the first time. Result: in the Wedding Video I made, during the Best Man's speech, he is relating a story about his brother (the groom) breaking his surfboard in half and swearing at the Atlantic Ocean whilst hitting it with his broken surfboard. While he is saying all that
a superimposed video appears of the groom doing exactly that - a video I took 14 years previously! Moreover, the wedding video is in 16:9, but the inset clip is in its original 4:3 aspect!
Cool or what!
Have fun!
