"What did you take pictures of at the park?"
Translation:Що ти фотографував у парку?
"She took pictures at the airport" is accepted.
"What did you take pictures at the park" isn't accepted. Why is "of" necessary here? Explain please.
In the first sentence, you are giving the location of her activity, not the object in her pictures. You could say, "She took pictures of airplanes at the airport." In the question, the park is the location, but the question is about the object in the pictures, so you need to use "of".