You
can use the Save As command to save CMYK, RGB, and grayscale images
in JPEG format. JPEG compresses file size by selectively discarding
data.
Note: You can also save an image as one or more JPEG files
using the Save For Web & Devices command.

To
quickly save a medium-quality JPEG, play the Save As JPEG Medium
action on the file. You can access this action by choosing Production
from the Actions palette menu.