I had this problem repeatedly and it was making me mad hatter mad. I did finally figure out what the problem was I think though.

I tried rebuilding from scratch several times and as soon as I imported the images, I'd start getting this error message. I thought I was going to have to rebuild all of my graphics and was honestly feeling sick to my stomach. Thought about changing careers.

But!!

As I was importing .psd's into my flash file, I noticed that some of them had a blank layer. I never really thought it would cause a problem.. it's blank. But, I deselected that blank layer as I migrated images in that had one, and never had the problem again.

I really think this blank layer caused confusion with flash. It didn't know what to do with it. The layer had no instructions..

Anyway, hope it helps someone.

~ Stef