There are some issues with this technique:

• Mobble uses (whisper it) UA sniffing
• It only works for featured images not inserted images
• It’s reliant on website editors using featured images
• Only one image per page

So…

I’m working on a plugin that will use JavaScript to detect the screen width then send a variable to a PHP file which will then automatically reduce image file sizes for all images, not just post thumbnails.

Anyone want to help me?

Finally…

If you liked this, keep an eye out for my upcoming Smashing Magazine articles and my book, ‘WordPress Mobile Development Beginner’s Guide‘ – published by Packt Publishing.