Tips from an old issue of .NET...

If your image is a JPEG, double-click it in the library, check the 'Use imported JPEG data' box and uncheck 'Allow smoothing'.

If it's a GIF, uncheck 'Allow smoothing' and make sure it has a one pixel transparent border (?!!)

I've used Flash a little at work but never noticed imported graphics being degraded. Maybe this only applies to an earlier version of Flash?