Google account mandatory for sharing photos via new feature
While the messaging feature is interesting and easy to use, it is imperative to note that the receiver should have a Google account with Google Photos installed to receive the sent image/message.
It is the same drill that goes with any other messaging app, but it shouldn't be a problem given that Google Photos already has more than a billion users.
Google made a similar move with YouTube too
Google had launched a similar feature for the YouTube app but the capability was killed a year later.
The company has high hopes for the idea of Photos-based messaging, but it still remains to be seen how the public receives it, considering the largely unsuccessful run of the company's previous social/messaging products and the dominance of WhatsApp and other Facebook products in messaging/media-sharing arena.