I found a photo on the internet and want to use it in my free Android app.
The problem is that I can’t find the source, so I don’t know if this photo is legally available for such use or not.
What is a sensible course of action when a designer wants to use a photo / graphic, but is unable to identify its source?
Try using Google’s search by image to find the original.
If you are unable to find it or the copyright about using it, you should not use it in your app to be safe.