^ Don't let them see you take a picture. And try to get a picture of them doing something. Because if you only take pictures of egg yolks...the authorities could accuse you of deliberately throwing the eggs to make it seem like they did. So, snap a photo of them in the act.
Who are you going to show the evidence (photos) to if the police seems apathetic? Maybe the police was apathetic because you had no evidence therefore it just made the scenario seem more like a "he said, she said tattle-tale" story. So, this time you should be more ASSERTIVE with the police. Show them the evidence (make multiple copies of phots).....in case the police gets rid of your pictures. And if the police offer you speak to is not helpful....then tell him "I want to speak to the officer in AUTHORITY over YOU"