將一幅圖片按著需要的尺寸進行等比的壓縮和放大，最後再截取需要尺寸部分，不知道說清楚沒，反正就那意思吧！

+(UIImage *)compressImageWith:(UIImage *)image width:(float)width height:(float)height
{
float imageWidth = image.size.width;
float imageHeight = image.size.height;

float widthScale = imageWidth /width;
float heightScale = imageHeight /height;

// 創建一個bitmap的context
// 並把它設置成為當前正在使用的context
UIGraphicsBeginImageContext(CGSizeMake(width, height));

if (widthScale > heightScale) {
[image drawInRect:CGRectMake(0, 0, imageWidth /heightScale , height)];
}
else {
[image drawInRect:CGRectMake(0, 0, width , imageHeight /widthScale)];
}

// 從當前context中創建一個改變大小後的圖片
UIImage *newImage = UIGraphicsGetImageFromCurrentImageContext();
[newImage retain];
// 使當前的context出堆棧
UIGraphicsEndImageContext();

return newImage;

}