Have a look at the Apple naming convention in their "Resource Programming Guide" on page 46.
The bundle- and image-loading routines automatically look for image
files with the @2x string when the underlying device has a
high-resolution screen. If you combine the @2x string with other
modifiers, the @2x string should come before any device modifiers but
after all other modifiers, such as launch orientation or URL scheme
modifiers. For example:
MyImage.png - Default version of an image
resource.
MyImage@2x.png - High-resolution version of an image
resource for devices with Retina displays.
MyImage~iphone.png -
Version of an image for iPhone and iPod touch.
MyImage@2x~iphone.png -
High-resolution version of an image for iPhone and iPod touch devices
with Retina displays.