If you already have your file full of images (like I did) you can:
ctrl+click on the picture and "Save Image As"
save a .png to the desktop
then drag'n'drop those back into place on the xls document
save, and see how much you've compressed
Kinda tedious, but not as much as re-opening the files in photoshop and resizing (unless you know how to batch process files, but that's another thread I think).