Head to photos.google.com/albums, click an image within an album, and choose “Slideshow” from the overflow menu (upper right corner). The team writes,

You like crowding friends and family around your computer or TV to show them photos of last weekend’s adventure. You don’t like clicking 132 times to show them each photo.

Luckily for you, it’s now possible to play a slideshow from any album. Just open the album, click on a photo, and select Slideshow from the dropdown menu.

For the ultimate viewing experience, cast the Chrome tab to your TV while you sit back, relax, and make everyone jealous with your amazing photos.

Slideshows are supported in Photos on Android as well.