What version of Flash? What format are your graphics? How did you create them? I've never seen this in many years working with Flash on many OS's, so it seems it could be something specific to how you work.
- Flash Pro CS5.5
- JPEG images
- File > Import > filenames.jpg
- also the images are fairly large, 1100 px in one of the dimensions.
My best guess is that you're somehow bumping up against a size limit like this Size limits | SWF, bitmap files | Flash Player 10. I know you're not using the Flash Player, but it's possible that since that limit is known there are limitations in the IDE that are related to it. Have you tried splitting your images into smaller pieces and combining several in the same Graphic symbol?