What is the resolution and dpi of the images you use for mobiles and tablets?

I know you should swap about images for optimized mobile display, what resolution(and dpi) should I use?

I imagine I should use retinaJS for retina ready devices, and 300 dpi images for that. Also the best method is to swap with JS?

Epic visual tool

Epson 5030UB 2D/3D 1080p 3LCD Projector Submitted September 28, 2015 at 06:13AM by kuncogopuncogo http://ift.tt/1Gbq3qR