Why we need #responsive images - TimKadlec.com
The topic of #responsive_images has been one of the most hotly debated topics amongst web developers for what feels like forever. I think Jason Grigsby was perhaps the first to publicly point out that simply setting a percentage width on images was not enough, you needed to resize these images as well. He showed that if you served appropriately sized images on the original responsive demo site, you could shave 78% off the weight of those images (about 162kB) on small screens.
The discussion has evolved since then with debates over what sort of solution we need (server-side, client-side), new markup (srcset vs picture) and even, in some cases, wondering whether we really needed to worry about it at all.