that's a common misconception , they actually have the rights to modify, publish etc. as long as the photos are on your account. otherwise, they couldn't even make a thumbnail out of it.
but if and when you delete them from your account, they have no rights at all. also, they could have rights to sell photos as microstock for all I care, first time they used that right, the best picture on all of facebook would be a puppy playing with a toddler at sunset shot by a mom from virginia. so I doubt they would ever exercise those rights

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