Download an Image and Save it as PNG or JPEG in iPhone SDK

code : 

NSLog(@"Downloading...");
 // Get an image from the URL below
 UIImage *image = [[UIImage alloc] initWithData:[NSData 

dataWithContentsOfURL:

[NSURL URLWithString:@"http://www.objectgraph.com/images/og_logo.png"]]];

 
 NSLog(@"%f,%f",image.size.width,image.size.height);
 
 // Let's save the file into Document folder.
 // You can also change this to your desktop for testing. (e.g. 

/Users/kiichi/Desktop/)
 // NSString *deskTopDir = @"/Users/kiichi/Desktop";
 
 NSString *docDir = [NSSearchPathForDirectoriesInDomains(

   NSDocumentDirectory, NSUserDomainMask, YES) objectAtIndex:0];

 
 // If you go to the folder below, you will find those pictures
 NSLog(@"%@",docDir);
 
 NSLog(@"saving png");
 NSString *pngFilePath = [NSString stringWithFormat:@"%@/test.png",docDir];

NSData *data1 = [NSData dataWithData:UIImagePNGRepresentation(image)];
                [data1 writeToFile:pngFilePath atomically:YES];
 
 NSLog(@"saving jpeg");
 NSString *jpegFilePath = [NSString stringWithFormat:@"%@/test.jpeg",docDir];
 NSData *data2 = [NSData dataWithData:UIImageJPEGRepresentation(image, 

1.0f)];//1.0f = 100% quality
 [data2 writeToFile:jpegFilePath atomically:YES];
 
 NSLog(@"saving image done");
 
 [image release];

important link:

http://blog.objectgraph.com/index.php/2010/04/05/download-an-image-and-save-it-as-png-or-jpeg-in-iphone-sdk/