Here is the copyright violation form link that facebook has COMPLETELY BURIED deep in their help FAQ about copyright violations, copied here for your easy reference:

You can only report your OWN work. You can’t report work on behalf of other artists who you know who someone has stolen from. If you think that artist will want to file a report, please alert them to the theft.

As henna artists, we experience a lot of image theft. It’s up to you to decide what to do about it. But if you’re going to do something/anything after your images have already been stolen, sending copyright violation notices directly to the hosts of those images is the most effective way to get the stolen material removed from the infringing site.