How do I get ex back?

So I was out with a group of friends and one of my friends who is a girl jumped on my back as a joke. I gave her a piggyback ride. Someone took that pic and posted it on Facebook.
My girlfriend saw that and she was really upset. She said it was wrong of me to do that. I explained it to her that I have no feelings for my female friend and we were just joking around. She just got really angry and broke up with me. It’s been one week.
I deleted that pic and stopped talking to my female friend. What should I tlel my ex?
How do I get ex back?
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