Here you have an example of saving the image:
assuming you get the content of the image into NSData object:
NSData *pngData = UIImagePNGRepresentation(image);
then write it to a file:
NSArray *paths = NSSearchPathForDirectoriesInDomains(NSDocumentDirectory, NSUserDomainMask, YES);
NSString *documentsPath = [paths objectAtIndex:0]; //Get the docs directory
NSString *filePath = [documentsPath stringByAppendingPathComponent:@"image.png"]; //Add the file name
[pngData writeToFile:filePath atomically:YES]; //Write the file
Have a look into these questions for more detailed explanations:
Save An Image To Application Documents Folder From UIView On IOS