Android photo viewer that doesn't blank screen
I have looked for that kind of app in all my installed apps but no photo viewer requests such permission. I really doubt there is a photo viewer that keeps your screen awake, because of battery drain (which is not exactly what an average user would be looking for)

Maybe what you should do instead is to search in Google Play for an app made specifically for keeping your screen awake, and use it with your favorite photo viewer. There are lots of them, and many are free, like this one (Keep Screen On)