Responsive Images - Approach A

The following image will automatically load either a small or large version depending on screen resolution, as long as cookies are enabled (non-cookie users will get the mobile size). This approach assumes your small and large image filenames are predictable (such as filename.jpg vs filename.large.jpg). If you don't have this flexibility, see Approach B

Read more about this here: Responsive Images 2 Readme

Photo credit (CC): Cia de Foto Flickr Creative Commons