Here's how to go about doing this:
UIImageView *imageView; // ...image view from previous code
NSData *imageData = UIImagePNGRepresentation(imageView.image);
PFFile *file = [PFFile fileWithData:imageData]
[file saveInBackground];
…and to retrieve it again:
[file getDataInBackgroundWithBlock:^(NSData *data, NSError *error) {
if (!error) {
imageView.image = [UIImage imageWithData:data];
}
}];