You should always keep your advertising images fresh.

If you shoot a lot of client boudoir, you can ask them for a model release on an image you want to use. DO NOT use without their permission.

If you don't have any to use - start shooting. Get models who will give you a release  for a free shoot and some images.

You need images that Facebook will approve for a paid ad. Their rules are a lot stricter for ads than they are for posts. If you show to much skin - your denied. If you show too much cleavage - your denied. So the trick here is to use images that say boudoir, without being too sexy.

Good Luck - Let's get shooting