You can block your Facebook photos from being shared with anyone outside of your own network -- so if your MIL shares the picture, the only people that can see it are mutual friends. You cannot, however, stop her from doing a save as and then reposting to her Facebook page. Ask me how I know this…
You can also limit which of your friends sees what posts. You can create a group with just your ILs and other family members in it and make sure they are excluded from those posts.
Or you can just limit what you post - which is what we've started doing b/c m ILs are retired and respond to anything we post on FB within 60 seconds.