It’s always best to use your own photos if you can, showing your actual church members. Remember to get permission of course, especially if pictures include children, and make sure you follow your church’s safeguarding procedures.
But sometimes stock photos are really useful, and there are several sources of free photos you can use. Some of these require attribution although most do not. These are the ones I go to most often:
It’s easy to find pictures using Google image search, but be careful – most images are copyrighted. You can filter by licence to look for ones which can be reused, but you will need to attribute the photo properly unless the licence terms explicitly say you don’t need to.