One of the new features introduced alongside Apple Music and the iCloud Music Library in the latest versions of iTunes and iOS is the ability to now specify an image of your choice to represent any of your playlists. By default, iTunes and iOS will simply use a collage of the album artwork from the tracks in your playlist, but you can actually add any photo you want instead.
In iOS 8.4, this is done by opening the playlist, tapping the “Edit” button, and then tapping the artwork, at which point you’ll be prompted to either choose an image from your photo library or take a photo with your device’s camera.
You can also do this in iTunes while editing a playlist by clicking on the artwork image. On OS X you’ll get the same image picker used for contact photos, allowing you to choose one of the stock photos, browse for an image on your Mac, or use your Mac’s camera to take a picture. In either case, the image you’ve chosen will sync with the playlist between your devices — but be aware that once you’ve done this, there doesn’t appear to be any way to get back to the default album artwork collage, as there’s no “remove image” option to be found anywhere.