How do I ask my girlfriend for feet pics?

So unlike the title suggests (I couldn’t find a better way to word it) I have asked my girlfriend recently if she was willing to take and send me feet pics and I was very happy to hear a yes and that she was willing to. My only concern and question that I have is how does one go about asking their partner for feet pics in a totally casual way? I know it’s different from partner to partner so if any of you have any tips for when you first asked so I can apply them to my situation as I would love to find a way to ask that isn’t too strange if that makes any sense.