Gaslighting is a form of psychological manipulation in which a person seeks to sow seeds of doubt in a targeted individual or in members of a targeted group, making them question their own memory, perception, and sanity.
I got that from Wikipedia. You can lie to someone and not care if they believe you or not. Specific to gaslighting is not only the lie, but doing something to make the other person question if they are incorrect or misunderstood. If maybe the lie is accurate.
If the question is "what happened to the picture" the answer "I don't know" can be a lie. The answer "What picture, there was never a picture" is trying to make you question your own recollection of the situation.