I don't think it's possible -- Sparky pays for disk storage, whether the photo is posted for all to see or just for one recipient. Just as he's made a decision to limit the size and number of pictures we post, he limits that posting to publicly-viewable posts.
Because -- a picture may be worth 1000 words, but it takes up the space of 20,000.
I'd suggest that if you want to share a picture you either get someone's email address, or you post the picture on a photo- or file-sharing site (like Google Drive) and share a link to the picture in the PM.