So far, I have been making wallpaper apps using content from google images with “labelled for reuse” selected.
I see youtube apps also, which seem to display copyrighted content, e.g. Metallica Videos.
Anyways, how should I best go about sourcing material without getting banned?
There are several websites with images etc. that are either public domain or released for royalty free commercial use. Often the licence terms do require a link back to the site to be included in your app.