What does it mean when a girl asks to see a picture of your girlfriend?

It means she wants to see a photo of your girlfriend. There are millions of potential reasons for someone asking such a question. If you want to know why the girl asking to see a photo of your girlfriend is asking then ask her why she wants to see said photo. Communication isn't as difficult as some people seem to think. If you want to know why, then ask why. That doesn't mean she's going to answer honestly.

Also, and this is really important, think about what reasons she might have to ask that you might prefer. If it's because you like her and may want to date her rather than your girlfriend and want to see if she feels the same then you probably should assess if you're being fair to your girlfriend or if you need to discuss your relationship. After all, if you're thinking of ditching her for this person there must be something you're missing from your current relationship.

If on the other-hand you're not interested and you find her asking to be off-putting or think she's going to say something ignorant about your girlfriend then proceed with caution. If she begins to be rude then let her know that you don't appriciate it and would like her to stop. Set firm boundaries when it comes to how people treat you or those you care about.