What resolutions of source images and icons should the designer make for Retina? 300, 400 or with a margin of 500 ppi?

Question

Adaptive design

What resolutions of source images and icons should the designer make for Retina? 300, 400 or with a margin of 500 ppi?
For a responsive site.

Answer 1

No stock. The designer must draw these icons in vector.
Most browsers today support SVG icons and fonts very well. Example: Font awesome
Even if without SVG, it is already the layout designer's task to convert vector icons into png images of any desired size.

Answer 2

А фотографии?