Creep shot
From wipipedia.org
A creep shot is a photo taken surreptitiously and without permission of a person or part of a person whom the photographer finds attractive. For example, it might be of the breasts of someone in a tight top, the bottom of someone in a tight miniskirt or of a woman in a bikini.
Such photography is unlikely to be illegal, although it may be annoying to the other person and if done persistently might be regarded as a breach of the peace or harassment.