Go to Settings > Privacy > Photos. If the app appears there, turn it on.
If not, open the app and try to access or post a photo. It should ask you for access. Say yes. You can always go back to the privacy settings and turn it off if you change your mind.
Go to the Privacy tab in your Settings app and press on the line titled "Photos." Ensure the app(s) you want to give photos access to is turned on:. . .