Does my action of breaking off the engagement after learning of my fiancée's previous relationship constitute slandering chaste women (qadhf al-muḥṣanāt), and what is the expiation for that if I am mistaken?

The Shari'ah has decreed the obligation of verifying the report of a fasiq (transgressor), based on the Almighty’s saying: "O you who have believed, if there comes to you a disobedient one with information, investigate, lest you harm a people out of ignorance and become, over what you have done, regretful." This implies accepting the report of a trustworthy person without verification. If what was attributed to the girl reached you through trustworthy individuals, there is no error in your informing her. However, your claim that he sent you a picture of them is a lie, and you must repent from it. What you mentioned about the girl is not among the slandering of chaste women (qadhf al-muhsanat), as that refers to accusing a chaste Muslim woman of adultery.