How to dump this guy in a mean way?

I just found the pictures of my boyfriend hugging other girl in his hardrive a few days ago. He left me his hardrive & his belongings before & he left out of town to work. The picture was taken around mid February this year. In the pictures, He was wearing a polo shirt which i brought him in Jan as his birthday gift this year and He hugging the girl around her shoulder and the other picture is my boyfriend siting at a certain height the girl standing on the ground inbetween my boyfriend leg and with his arm around her neck with the polo shirt i brought him. And they look very happy in the pictures and they are so close like couples.. Actually, We planing to get marry but i feel very disappointed after i saw this pictures and am rethink should i marry to this guy or not? i chose to forgive him many things in the past if he did anytime wrong. He hurts me more after i see the pictures. He's like taken me for granted. i am really upset. Should i change my number for a new life? Would someone help on this?