方法1:
```//sdk中提供了方法可以直接调用
UIImage *img = [UIImage imageNamed:@"some.png"];
NSData *dataObj = UIImageJPEGRepresentation(img, 1.0);
//下面是sdk中UIImage.h头文件中的内容
UIKIT_EXTERN NSData *UIImagePNGRepresentation(UIImage *image);
// return image as PNG. May return nil if image has no CGImageRef or invalid bitmap format
UIKIT_EXTERN NSData *UIImageJPEGRepresentation(UIImage *image, CGFloat compressionQuality);
// return image as JPEG. May return nil if image has no CGImageRef or invalid bitmap format. compression is 0(most)..1(least)
```
JPEG的转换方法里面第二个参数是压缩系数，可以有效的减小图片的大小。UIImagePNGRepresentation(UIImage* image) 要比UIImageJPEGRepresentation(UIImage* image, 1.0) 返回的图片数据量大很多。项目中做图片上传之前，经过测试同一张拍照所得照片png大小在8M，而JPG压缩系数为0.75时候，大小只有1M。而且，将压缩系数降低对图片视觉上并没有太大的影响。
方法2:
```
- (UIImage*)scaleImage:(UIImage*)image scaleFactor:(float)scaleFloat{
CGSizesize =CGSizeMake(image.size.width* scaleFloat, image.size.height* scaleFloat);
UIGraphicsBeginImageContext(size);
CGContextRefcontext =UIGraphicsGetCurrentContext();
CGAffineTransformtransform =CGAffineTransformIdentity;
transform =CGAffineTransformScale(transform, scaleFloat, scaleFloat);
CGContextConcatCTM(context, transform);
// Draw the image into the transformed context and return the image
[imagedrawAtPoint:CGPointMake(0.0f,0.0f)];
UIImage*newimg =UIGraphicsGetImageFromCurrentImageContext();
UIGraphicsEndImageContext();
return newimg;
}
```