Been looking for something like this for a long time!
“TinySrc is now a part of Sencha.io. It will remain a free service for single users, known as Sencha.io Src. Sencha.io Src performs smart sizing of images for efficient mobile delivery.
Sencha.io Src helps you dynamically resize images for the ever increasing number of mobile screen sizes. We’ve previously done a lot of work in Sencha Touch to make your UI resolution independent, and Src expands this to include your image assets. It’s easy to use, and in this guide, we run through the main API options for the service.
Sencha.io Src is essentially a proxy that lies between image assets (hosted either on your own server or by a third party) and the browser or application requesting them via HTTP. The API is accessed entirely via placing a prefix before the original image URL. This prefix gives you declarative access to all of the different types of transformation that the service can perform. This approach makes the service very easy to add to existing web sites or apps without any programming knowledge.”