Proper responsive images—sending different image files to different devices

Web developers have been working on the problem of sending different files to different devices for a few years, and a number of solutions have been proposed. Out of the plethora of methods out there, I'm going to look at one which uses inbuilt WordPress functionality. This method uses WordPress-featured images combined with a plugin called mobble to identify mobile devices.

However, before we start it's important to address one assumption about mobile connections.

Are mobiles always slow?

It's easy to assume that mobile users will be on a slower connection than desktop users, but this may not always be the case. Consider the growing number of smartphone owners who use them to access the Internet at home.

Note

A survey of smartphone users (http://blog.compete.com/2010/03/12/smartphone-owners-a-ready-and-willing-audience) found that 84 percent of them use their phone to access the Internet at home, making this the most...