对于jpg图片来说，有损压缩因子设置为0.5 可以大大减少图片的体积，而对图片的质量几乎没有太大影响：

下面是测试图片结果：

//    UIImage *image_jpg = [UIImage imageNamed:@"jpgtest.jpg"];

//    UIImage *image_png = [UIImage imageNamed:@"pngtest-1.png"];

//

//    NSData *data_pngfrompng = UIImagePNGRepresentation(image_png);

//    NSData *data_pngfromjpg = UIImagePNGRepresentation(image_jpg);

//    NSData *data_jpgfrompng = UIImageJPEGRepresentation(image_png, 0.5);

//    NSData *data_jpgfromjpg = UIImageJPEGRepresentation(image_jpg, 0.5);

//

//    NSString *documents = [NSHomeDirectory() stringByAppendingString:@"/Documents/"];

//

//    NSString *pathpngfrompng = [documents stringByAppendingString:@"data_pngfrompng.png"];

//    NSString *pathpngfromjpg = [documents stringByAppendingString:@"data_pngfromjpg.png"];

//    NSString *pathjpgfrompng = [documents stringByAppendingString:@"data_jpgfrompng.jpg"];

//    NSString *pathjpgfromjpg = [documents stringByAppendingString:@"data_jpgfromjpg.jpg"];

//

//    [data_pngfrompng writeToFile:pathpngfrompng atomically:YES];

//    [data_pngfromjpg writeToFile:pathpngfromjpg atomically:YES];

//    [data_jpgfrompng writeToFile:pathjpgfrompng atomically:YES];

//    [data_jpgfromjpg writeToFile:pathjpgfromjpg atomically:YES];

UIImage *image_jpg = [UIImage imageNamed:@"2231249403.jpg"];

NSData *data_jpgfromjpg = UIImageJPEGRepresentation(image_jpg, 0.5);

NSString*documents = [NSHomeDirectory()stringByAppendingString:@"/Documents/"];

NSString *pathjpgfromjpg = [documents stringByAppendingString:@"datcccc.jpg"];

[data_jpgfromjpg writeToFile:pathjpgfromjpg atomically:YES];

NSLog(@"documents=%@",documents);