A young girl in a black leotard swinging on the lower wrong of a pair of parallel bars in a gym while a young man spots her.
The young girl is wearing a black leotard
A young girl in a leotard is wearing it.
A girl wearing a leotard is the same as a young girl being in a leotard.
"A young girl in a black leotard" is a way of saying "the young girl is wearing a black leotard."