OK, I guess you also checked the QuickTime plugins? Although that may not help anyway, given that they are located at the top level (HD/Library/QuickTime) - not in a user account. But check to see if there is a QuickTime folder in your User/Library folder (there shouldn't be under Snow Leopard).
Also, do a Spotlight search for Flip4Mac and Perian. Search also for GLTools.plugin - this is not QuickTime related but can be found on some systems in the folder HD/Library/Graphics/Quartz Composer Patches. This file has been known to cause problems with iMovie (mainly with Titles). According to reports on the forum, it can be safely deleted (it doesn't exist on my system).
Another suggestion: We haven't heard what format your photos are. Whatever they are, drag one to the Desktop from iPhoto. Open it in Preview and Save As a jpeg (with a different name) to the Desktop. It may already be in that format, but I've found that re-saving in this way can sometimes allow it to work nicely with iMovie - this also works when photos won't import to iPhoto. I've found that some photos generated by 3rd party programs, even though jpegs, don't always work well with some Mac applications. Re-saving in Preview seems to fix it.
After re-saving, re-import the photo to iPhoto and see if you can drag it from iMovie's media panel and drop to your project. Also, try dragging it from the Desktop.
You have of course tried shutting down and rebooting? Sorry to ask this but it's often overlooked - a simple reboot often fixes glitches in iMovie (as does closing and re-opening iMovie).
John
PS. Some of this information may appear irrelevant (such as the photo re-save trick), given that things work OK under a new user account. But the suggestions are worth trying - you never know!
Message was edited by: John Cogdell - added PS and corrected typos.