Hmmm, if you're talking about reducing the file size WITHOUT affecting the image dimensions I have a tip (I do this all the time btw). Open the file using Windows Paintbrush, for default Windows installations just right-click the file and select "Edit". Once the file is loaded, immediately save the file under a NEW FILENAME ELSE THE EXISTING FILE WILL BE OVERWRITTEN. The saved file will look the same, have the same dimensions and EXIF data, but the file size will be significantly smaller.
Warning and please note:
1) Only do this if you're not gonna do any more editing or Photoshop as after saving via Paintbrush the effects of editing the photo will be very different.
2) I suggest you copy the photos to another directory as one wrong click and you'll ruin the photo forever. You can't undo the Paintbrush save once its done.
Hope this helps!