How to choose the image format and sizes for Ipad nonretina and I pad retina?

Currently, I am making 2d game for Ipad nonretina and Ipad retina. But I dont have clear idea of choosing image format and how to handle images for different resolutions. Do I have to have different (1x, 2x) size images, and what is the proper image size of background image?

Any help would be appreciated. Thank you

Check out the use of Screen.height and Screen.width