She wants to compare herself to your new girlfriend. I would not send her a photo as she might [if she is that way] make nasty , horrid remarks about her. It is normal for a ex to want to know what the new girlfriend looks like but like i mentioned it is to compare looks etc.
She probably wants to see her picture in hopes that she can make herself feel better about the situation. I know I got a lot of satisfaction when a guy who hurt me started dating a girl that wasn't "as pretty," and everyone knew it/ talked about it.