I've always called images the "ugly child" of RWD. Why? Until the last minute, I always tried to avoid having to deal with them. Do I use image sprites? If so, do I export my transparent PNG as an 8 bit or 24 bit, or 32 bit? Some legacy IEs don't support PNG with alpha channel, so I have to export a GIF sprite. I can use SVG instead, but IE8 and below don't support SVG. I can use icon fonts, but what happens if the icon font doesn't load? I'm going to have to look up some analytics then. There's a new iDevice with a new type of high-density screen? Now I have to export two (or more) images every single time. Great! But I can't serve a high-quality image that's more than double the size of the regular size image to small-screen devices! Yeah, it may look good but it'll take forever to download, they just might as well leave the site even before the first H1 loads.
You get the picture. That's barely scratching the surface of...