(As odd CFNFs go, this is in my "Top 5"!)
Take your time and soak in all the details. The most logical explanation I have for this one is that it is another one of those "Nude Bicycle Events" but there's not a bike to be seen and those usually have crowds in the frame. Short of that? This is well into 'WTF-land'.
-obvious age discrepancy but they don't seem related (but could be, I suppose)
-it's definitely public and not a private backyard
-there's a clothed person walking by in the background
-the clothed woman is wearing a dress as opposed to comfy jeans and a tee which for me suggests this is some kind of 'event' (again one of those bicycle events seems most likely)
-their pose is comfortably loving and friendly, so this doesn't seem to be a photo-op for a curious clothed onlooker. More likely they know each other well or ARE related
-the naked girl seems to be barely 18 and therefore unusually young for this to be a 'proud mom' photo...........but the clothed woman appears to be very proud
-the naked girl is just adorable!......and cute as a button...........but that doesn't really offer a clue. It's just a personal observation.
So, what do you really think this is all about? (And if the word "spank" appears in your comment, I swear I am sending you to Merry hoping she has some thorned switches leftover from Valentine's Day! There is NOTHING in this shot to suggest anything to do with spanking.)