You're asking the wrong question. It's not up to you to prove the unlawfulness, it's up to them to prove the lawfulness (which is clearly not given if the contr

You're asking the wrong question. It's not up to you to prove the unlawfulness, it's up to them to prove the lawfulness (which is clearly not given if the contract expressly mentions only damage photographs). The next time this happens, simply tell them that they have no right to photograph beyond what was agreed, and instruct them to cease immediately. If they feel differently, it is up to them to demonstrate on what grounds they may photograph.