Windows & Mac (Chrome or Edge)
Click the Install icon at the right end of the address bar (a monitor with an arrow), or the button above. The app then appears in the Start menu / Launchpad.
Android
In Chrome, open the menu (⋮) › Install app or Add to Home screen.
iPhone & iPad
In Safari, tap Share › Add to Home Screen.
Mac (Safari)
In Safari, choose File › Add to Dock.
The app is the same as the website: your pictures stay on your device.