However, it's more than just preventing photos. It's about respecting boundaries. At some point your parents need to respect your authority in your children's lives. They're your children, not your parents children. Ultimately you are responsible for their wellbeing. They should be supporting your parenting, not undermining it.
This, obviously, is one issue among many, and may not be representative of a broader pattern. From your hesitation it sounds like this is one disagreement in an otherwise healthy dynamic. Again, you know better than us. But if it is a pattern, I encourage you to play that pattern forward. If your parents don't respect your boundaries, what does it look like if your parents don't respect your children's boundaries as they grow?