Is merely imagining the Prophet, may God's prayers and peace be upon him, while laughing at a story considered an act of disbelief?

Your laughter is not disbelief (kufr), for not all laughter is mockery. It seems you are afflicted with obsessive whispers (waswasah), so ward them off. We advise you to supplicate and humble yourself, to distract yourself from these whispers, and to consult a trusted psychiatrist.