Load UIImage from file using grand central dispatch

I'm trying to load images in the background using gcd. My first attempt didn't work:

dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_LOW,0), ^{
   dispatch_async(dispatch_get_main_queue(), ^{
     // create UI Image, add to subview
   });
 });
 

commenting out the background queue code and leaving just the main queue dispatch block didn't work:

dispatch_async(dispatch_get_main_queue(), ^{
   // create UI Image, add to subview
 });
 

Is there some trick to doing ui stuff inside a gcd block?

In response to mattjgalloway's comment, I'm simply trying to load a big image in a background thread. Here's the full code that I tried originally:

dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_LOW,0), ^{
 
     UIImage* img = [[UIImage alloc] initWithContentsOfFile: path];
 
     dispatch_async(dispatch_get_main_queue(), ^{
 
        imageView = [[[UIImageView alloc] initWithImage:  img] autorelease];
        imageView.contentMode = UIViewContentModeScaleAspectFill;
        CGRect photoFrame = self.frame;
        imageView.frame = photoFrame;
        [self addSubview:imageView];
 
   });
 
 });
 

I simplified it so that I was running everything inside the main queue, but even then it didn't work. I figure if I can't get the whole thing to work in the main queue, no way the background queue stuff will work.

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