How to give an app access to photos on iPhone?

I know you're supposed to go to your settings and then photos and turn the access to "on" but the app isn't listed in the settings at all. How to you give an app access to your photos when it's not listed in your settings?

You missed a step. Go to Settings/Privacy/Photos. From there, you will find the app you're looking for. Switch it from OFF to ON.

And if the app is not listed at all, how does the app get listed in this area? I'm having trouble with iOS7 now that I've upgraded

I have the app but all of a sudden it keeps telling me it needs access to my photos. It alwYs worked until a few days ago. Any suggestions?