MD5 Checksum with NSData

UIImage* im = [UIImage imageWithCGImage:cgimg]; // make image from CGRef

NSData* pixelData = UIImagePNGRepresentation ( im ); // get PNG representation

if (pixelData)
NSLog(@"%@", [pixelData MD5]);

http://iphonedevelopertips.com/core-services/create-md5-hash-from-nsstring-nsdata-or-file.html