图片加载优化~

There are some use cases (not the majority) for which the CALayer and Core Animation approach will not work. For those cases, keep in mind that CGContextDrawImage and UIImage/-drawInRect: could spend a lot of time in decompressing or resampling the images as necessary because the images will not be cached in the layer tree. 

http://www.keakon.net/2011/07/26/利用预渲染加速iOS设备的图像显示