I found out the girl I am dating used to be fat so I dumped her.
That's awful. How did you find out she used to be fat?
She showed me a picture of herself when she was fat.
It's wrong to judge someone based on appearance, be it how the looked in the past or currently. This way of thinking is cruel and it also can prevent you from having a great relationship.
I never said I was perfect and I can have preferences.
It is one thing to have preferences, but it's another to be mean and judgmental. You should try to be more accepting of others and recognize that being dismissive and superficial is wrong.