【Bitmap Images and Image Masks】
Bitmap images and image masks are like any drawing primitive in Quartz. Both images and image masks in Quartz are represented by the CGImageRef data type. Keep in mind that a bitmap image is an array of bits at a specific resolution.
A bitmap image (or sampled image) is an array of pixels (or samples). Each pixel represents a single point in the image. JPEG, TIFF, and PNG graphics files. With the use of the alpha component, they can appear to take on a variety of shapes and can be rotated and clipped.
Quartz also supports image masks. An image mask is a bitmap that specifies an area to paint, but not the color. In effect, an image mask acts as a stencil to specify where to place color on the page. Quartz uses the current fill color to paint an image mask. An image mask can have a depth of 1 to 8 bits.
【Creating Images】
If you want to create a CGImage object from an image file that uses a standard image format such as PNG or JPEG, the easiest solution is to call the function CGImageSourceCreateWithURL to create an image source and then call the function CGImageSourceCreateImageAtIndex to create an image from the image data at a specific index in the image source. If the original image file contains only one image, then provide 0 as the index. If the image file format supports files that contain multiple images, you need to supply the index to the appropriate image, keeping in mind that the index values start at 0.
If you’ve drawn content to a bitmap graphics context and want to capture that drawing to a CGImage object, call the function CGBitmapContextCreateImage.
Several functions are utilities that operate on existing images, either to make a copy, create a thumbnail, or create an image from a portion of a larger one. Regardless of how you create a CGImage object, you use the function CGContextDrawImage to draw the image to a graphics context. Keep in mind that CGImage objects are immutable. When you no longer need a CGImage object, release it by calling the function CGImageRelease.
Functions for creating images:
【Creating an Image Mask】
原文：http://www.cnblogs.com/tekkaman/p/3565692.html