The toolbar at the bottom of the note editing screen has an add-image icon and a camera icon. Tap the image icon to choose a photo from your device, or tap the camera icon to take a photo on the spot and attach it.
Up to 15 photos can be attached to one note.
If the camera doesn't open, check in your device's "Settings" app that the camera permission for this app is set to "Allow" (see "I can't attach photos anymore" for details).