[Montellese]

For me it's client responsibility to manage cache of re-sized images.

You just need to provide correct HTTP cache headers and 304 answers based on change of original image cache.

Just talking from my side but dealing with Android devices with 6 different possible DPI so 6 different size for the same image in the same screen depending on phone, means if size are limited on server side they won't be very useful.

Well they are already limited now. You only get the image in the original resolution. The idea was to provide a few additional sizes like 256px width and 512px width (with adjusted heights). That way you don't get exactly the size you want but especially in case of e.g. 256px width you can still save a lot of bandwidth.

But yes it comes down to who is responsible for caching. The problem with HTTP caching headers is that our webserver doesn't really have that functionality i.e. we don't keep track of what has been requested by who and if that file has changed in the meantime.