Though the digital revolution generates billions upon billions of photos, it’s still missing the albums and shoeboxes of old printed images lurking somewhere in homes around the world. Alphabet Inc.’s Google is launching a PhotoScan app so you can capture them on your smartphone. And—Google hopes—store them in its cloud drive, too.

The app, available Tuesday for Android phones and Apple Inc.’s iPhones, uses your smartphone’s camera to take four photos of a printed picture, which it stitches together into one digital recreation.

The app is designed to be a competitor to services that ask you to send old printed photos or rolls of film off to be digitized, said David Lieb, product lead for Google Photos. There are dozens of these services, ranging from Memories Renewed and DigMyPics to your local Walgreens or CVS.

The problem with those options, Mr. Lieb argues, is they are all expensive and time-consuming. “We make the scanning quick and we do all the work sorting your photos, and the app is free,” he said.

Here’s how it works: You open the Google PhotoScan app and hold your phone above a printed picture. It recognizes the edges and places four virtual dots on the photo. As you move your phone to match up with the dots, the app captures four separate images. Why four? So that the app can stitch together a bigger image that is free of glare and aligned properly. You don’t want this to look like a picture of a picture, shot on your phone.