When you’re working in Photoshop and you want to show the image full screen without the Photoshop interface, just tap the F key twice, and Photoshop will go into Full Screen Mode. Then, press Command-0 (PC: Ctrl-0) to fill the screen with your image. To reposition it on screen, just hold the Spacebar and click-and-drag the image. The Tab key will show or hide your panels. If the rulers are on, they’ll show in each view mode, so press Command-R (PC: Ctrl-R) to hide them. 

Each time you press F, it will toggle from Standard Screen Mode to Full Screen Mode with Menu Bar to Full Screen Mode. Press F to get out of Full Screen Mode, not ESC (I know, you’ll be tempted). By the way, this is why Photoshop doesn’t go into full screen mode on Mac when you maximize it because it has its own full screen mode built in, and I like it that way.