1.) Make sure your users know how to use the missing images report. This report should be run prior to billing so you know which required images are missing before creating your invoices.
2.) If you have an image that belongs to multiple shipments you can simply right click the image in the Image List on any Image Viewer window and select copy. Then go out to a dashboard, highlight all of the shipments that image also belongs to, right click and select paste. Now that one image has been copied to all the shipments it pertains to!