if I join a family sharing plan, will I lose my data (pictures) if I downgrade my personal icloud account?

When you share an iCloud storage plan your data stays where it is and continues to be accessed via your Apple ID. Only the amount of storage available and the billing change. Also, you don't cancel your existing plan. See the following from Share iCloud+ with your family - Apple Support

Switch from your iCloud+ plan to your family plan

When you share iCloud+, any family members using the free 5GB plan are added to your family plan automatically.

If a family member is already paying for their own iCloud+ plan, they can choose to switch to your plan or keep their own and still remain part of the family. When they switch to the shared family plan, they get a refund for the remainder of their personal plan. They can't keep their current plan and use the shared family plan at the same time.