As far as I know there's no "OUTPUT CHANNEL" feature in Illustrator ( like the one in Photoshop),
BUT this might work: Rasterize the image (Object....Rasterize), then go to "Filter...Colors....Adjust Colors"
and move the level sliders as desired ( to lighten or darken). You could probably even create an automated action, if you had lots of pictures.
Hope that helps.
Nochmals viele Gr