Using dispatch_async to load images in background

I am trying to update a progress bar as my images load. I've read several answers here and tried formatting my code many different ways. I'm trying to read in the images and update the progress bar. Then, will all the images are loaded call the code to process them. The best result I've got was some code that works most of the time. However, if I'm dealing with a situation where it is pulling in a lot of images, I get weird errors. I think it is going ahead and running the continue code before all the images are fully loaded. When I remove the dispatch_async, the code works fine but the progress bar does not update.

func imageLocXML(il:String) {
    dispatch_async(dispatch_get_global_queue(QOS_CLASS_USER_INITIATED, 0)) {
        let url:NSURL? = NSURL(string: il)
        let data:NSData? = NSData(contentsOfURL: url!)
        let image = UIImage(data: data!)
        pieceImages.append(image!)
        self.numImagesLoaded += 1
        self.updateProgressBar()
        if self.numImagesLoaded == self.numImagesToLoad {
            self.continueLoad()
        }
    }
}