I just ran into this problem myself.

I discovered for me, it had something to do with moving my images around to a different folder.
(ie: My main flash file was in one folder.. but I had moved some images from a 2007/website/assets folder... to a 2010/website/assets folder.)

So fix it... I would go to the assets/images in question in my "library window", (you can tell what files are in question, 'cause the preview gives an error)
and then double clicked on the item... I then clicked on "update" (in the window that pops up)... and then using the browser was able to track down the proper location for the image.

Pretty much similar to what the previous two posters had written.