In case you didn't know, this is a pressing concern for developers that do care about performance of responsive websites. Take a look at http://www.w3.org/community/respimg/

Scott Jehl of Filament Group just delivered a talk at An Event Apart on Responsible Responsive Design, and released Southstreet, a suite of tools designed to make responsive design more performant. Check it out at https://github.com/filamentgroup/Southstreet