Is it forbidden to view previous pictures and conversations that took place between two people who committed a forbidden act and then repented, and is their deletion a condition for repentance?

You both did well by repenting, for in it is the closing of the doors of temptation. It is not permissible for you to keep old messages and pictures or to look at them, because they could be a means to temptation, and the Sharia (definition) came to block the means [to evil], especially concerning the interaction between a man and a woman. The Prophet, peace and blessings be upon him, said: "The adultery of the eyes is looking, and the adultery of the tongue is speaking, and the soul wishes and desires, and the private parts testify to that or deny it." And part of perfecting repentance is to get rid of these messages and pictures.