Use the JPEG format to save an image as highly compressed, 24-bit bitmap. Generally, GIF is better for exporting line art, while JPEG may be better for images that include a number of gradients, or embedded bitmaps.
Flash exports the first frame in the movie as a JPEG, unless you mark a different keyframe for export by entering the frame label #Static.
Use the controls in the JPEG panel of the Publish Setting dialog box to change the following settings:
Dimensions Sets the size of the exported bitmap image to the number of pixels you enter in the Width and Height fields. If you turn on Match Movie, the entries in the Width and Height fields have no effect and Flash makes the JPEG the same size as the movie. Flash ensures that the size you specify always has the same aspect ratio as your original image.
Quality Controls the amount of JPEG file compression used. Lower image quality produces smaller files, while higher image quality produces larger files. Try different settings to determine the best trade-off between size and quality.
Progressive Displays Progressive JPEG images. These images are displayed incrementally by a web browser and may appear faster when loaded over a slow network connection. This option is similar to interlacing in GIF and PNG images.