for enabling retina display in corona add the following lines in config.lua

application = 
{
	content = 
	{ 
		width = 320,
		height = 480, 
		scale = "letterbox",
		imageSuffix =
			{
				["-x15"] = 1.5,		-- A good scale for Droid, Nexus One, etc.
				["-x2"] = 2,		-- A good scale for iPhone 4 and iPad
			},
	}
}

and include images with twice and 1.5 larger than the original and named as image-x2.png and image-x15.png in the project folder