As a mobile user
I don't want to have to download a 2000px page banner on my mobile device
So that I don't end up with horrible data charges
To do this instead of specifying an image url via $wgPBImageUrl I'll probably want to specify a page in the file namespace so that images of different size can be generated.
Also set custom attributes like data-src-set="2x: http://w.co/foo800x;" on the element ".ext-wpb-pagebanner" to specify urls of standard banner sizes.