FROONT seems to be worth playing with.
Piotr Walczyszyn created nice tool in Responsive Inspector (Chrome Web Store link).
Nice overview how to code for high pixel density (160 PPI and up) displays.
In terms of images, the goal of web app developers is to serve the best quality images as efficiently as possible. This article will cover some useful techniques for doing this today and in the near future.
Good list, you may find some other resources in my Zootool bookmarks. Also, try searching “responsive tutorial” etc.
This piece of work by Zurb may come in handy.