Hum, no, I did not included any kind of legal documentation in my app. But I think that this a really good Idea.
At the moment I just want to release my app as Beta. I think I will add that information 
However, I feel like I’m missing something? I don’t have the feeling that adding this to my app will fixe my problem since Google is asking for a document that proves that I have the right to use the images (or any copyrighted content).
I looked at other applications on the store that uses also the images and they are not providing any “License” information.
I absolutly don’t know how to get an agreement document unfortunatly.
Do I need to register on https://devportal.twitch.tv/ first?
I still don’t know what to do with my problem 
Anyway, thank you for your help this is really constructive and appreciated.
If you have any other information, let me know
Please help 