A creep shot is a photo, taken surreptitiously and without their 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 leggings, or of a woman in a bikini.
Such photography is unlikely to be illegal if there is no specific violation of ordinary expectations of privacy, but this should be checked locally. However, it may be annoying to the other person and if done persistently might be regarded as a breach of the peace or harassment.
In 2012, publication of creep shots of a member of the British royal family topless caused considerable controversy and led to legal action.