Wow! Those are really awful.
When I see images like that, containing multiple chunks of images, my first thought is some kind of memory problem, where data gets corrupted in RAM — or while swapping data in RAM and virtual memory.
How much RAM does your Mac have?
Have you added any third-party RAM to your Mac or are you running Apple-only RAM?
Have you run any RAM diagnostics to find out if your RAM is flawed? I'd suggest memtest, available at versiontracker.com.
http://www.versiontracker.com/dyn/moreinfo/macosx/17156
But I'd first run Disk Utility to make sure the hard drive is functioning properly. You can Verify the drive in Disk Utility, but to Repair it — if problems are found — requires booting up your Mac from the Mac OS X Installer DVD.
There should also be a Hardware Test on one of the disks that came with your Mac.
Hopefully someone has seen a similar image and can pinpoint the cause.
Oh, be sure to rule out third-party QuickTime and iMovie add-ons that might be causing trouble for iMovie. More here:
http://discussions.apple.com/message.jspa?messageID=4030731#4030731
Karl