FYI, Progressive Web Apps (PWA) can be installed on Chrome for desktop and Android. You will soon start seeing more web apps come up. Perhaps the first high-profile PWA was the Google Maps Go, and now, Google has made another one high profile release by outing the Google Photos site (photos.google.com) to work as a PWA.
That means you can and Google Photos (the PWA version) as a shortcut to your home screen on your Android device. If you are on desktop, and accessing the internet via the Chrome browser, you should know Google has enabled PWA by default on Chrome 67, and it will probably remain that way going forward.
The key word being ‘should,’ some users report the feature is not turned on by default on their Chrome 67 browser. If you are one such user, you can turn on the feature by typing chrome://flags/#enable-desktop-pwas on your address bar then hit enter, the set Desktop PWAs to Enabled status by clicking on the drop-down button to the right.
After enabling PWAs, you will have to re-launch your browser for the changes to take effect. After re-launching the browser, you can go to photos.google.com and then click on ‘Settings’ (the three dots running downwards top right corner of the browser. In the jump list, click on Install Google Photos, and you will see a shortcut created on your desktop.
So far, the app does not appear to be using any of the fancy PWA features like offline support and push notification. There is a chance, of course, that will change in the future.