When you export your swf (shift+ctrl+alt+s) you’ll get a dialog box with various parameters - half way down is one called Jpeg quality which can contain a number between 0 and 100.
It’s a tradeoff between filesize and quality - the lower the number the smaller the filesize and the worse the quality. Try setting it to 100 and see what happens - then work back until you get an acceptable file size.
If you find that manipulating the jpeg compression at export doesn’t help then you have custom compressions applied to your imported graphics - this happens much more with imported PNGs than with Jpegs. Open your library, choose the offending pic, right click and choose properties uncheck the box that reads use default document quality and set the jpeg compression to 100
If your using png, or jpeg before you save and publish your movie, make sure you go File > Publish Settings
there you will find options for jpeg, make sure u also select the png option, that should make ur images come out more clearer.