Responsive Images: Use Cases and Documented Code Snippets to Get You Started

The new element is a great solution for many responsive image needs; here is a great reference for usage.

Andreas Bovens begins by asking four questions:

Do my image sizes change depending on my responsive design rules?
Do I want to optimize for high-dpi screens?
Do I want to serve images with different mime types to browsers that support them?
Do I want to serve different art depending on certain contextual factors?

Then he breaks down every permutation and gives code examples for each.