Yes, you can take pictures inside the Sagrada Familia and it would be a real shame not to. The basilica is actually the most Instagrammed monument in Barcelona ahead of Park Güell… You won't resist long either.
Can you bring water bottle into Sagrada Familia?
It is not permitted to take any food or drink into the Sagrada Familia. If you have some stashed in your bag, it's quite possible it will be taken from you as you pass through the security checks. If it isn't, you must refrain from eating or drinking while inside the basilica.