Although the facial recognition features in the iOS Photos app are pretty great, you may not always like the picture it chooses as the “key” photo for a given person. Fortunately, there’s a way to select a different photo, but it’s until recently it’s been so well hidden in the Photos app that most people probably don’t even know it’s there.
To select a new key photo for a person, you must begin by going to the “People” album in the iOS Photos app and selecting the person whose photo you want to change the image for. At the bottom of that person’s photos, tapping “Show Faces” will zoom in on the person’s face in each photo. To set a new key photo, tap the “Select” button in the top right corner, find the photo you want to use and tap on it to select it, and then tap the sharing button. Scrolling to the right in the bottom row of icons should reveal as button to “Make Key Photo” that you can tap on to set the selected photo as the key photo.
Note that the “Make Key Photo” option will not appear if more than one image is selected. Further, until very recent iOS versions, it was necessary to be in “Show Faces” mode before selecting the photo in order for the option to appear, but now it will work in either mode as long as only a single photo is selected.