On Tue, Jul 28, 2015 at 3:19 PM, Bernd Sitzmann <bernd@wikimedia.org> wrote:
1) leadImageWidth: 
1A) If the clients uses a constant value, let's say 800px for thumbsize action=mobileview parameter then the client could replace the /800px- portion in  the resulting URL with the desired width, as long as the URL structure stays predictable[2]. If the string replacement fails we could still use the 800px URL.

MediaWiki thumbnail URLs are fragile and not meant to be used as an API. Mainly because of T74328 but also because finding the size part is not always trivial (e.g. for a JPEG the URL could look like upload.wikimedia.org/wikipedia/commons/thumb/6/6e/Cernfounders.jpg/qlow-1024px-Cernfounders.jpg).