Screen Resolutions (in Pixels) by Noble Desktop

iPhone 3GS (Non Retina)

320 x 480 CSS

320 x 480 Hardware

iPhone 4 (Retina)

320 x 480 CSS

640 x 960 Hardware

iPad 2 (Non Retina)

768 x 1024 CSS

768 x 1024 Hardware

iPad 3 (Retina)

768 x 1024 CSS

1536 x 2048 Hardware

MacBook Pro 15-inch (Non Retina)

1440 x 900 CSS

1440 x 900 Hardware

MacBook Pro 15-inch (Retina)

1440 x 900 CSS

2880 x 1800 Hardware

iMac 21.5-inch (Non Retina)

2560 x 1600 CSS

2560 x 1600 Hardware

Graphics should be RGB and 72ppi. Resolution doesn’t really matter (that’s for printing, not screen display). Pixel dimensions (width & height) are what truly matter.
Retina screens (and graphics) are twice the width & height of normal screens, therefore we call them 2x.