renderInContext with iPhoto images working for PDFs but not thumbnails - objective-c

Question

I seem to have a curious bug involving renderInContext.
I am working on a "scrapbook" app which allows people to freely arrange text, colored squares and photos within a scrapbookView. Note that users can add both photos from a built-in collection of photos (loaded via imageNamed:) and photos from their asset library.
My code creates both PDFs and thumbnails as needed from the scrapbook view and all its subviews by first creating a PDFContext (for PDFs) or an ImageContext (for thumbnail images), then calling renderInContext on scrapBookView.layer.
In general, the code works, with one exception which I will describe. When I am creating a PDF, both built-in images and asset library images are correctly included in the rendered PDF. However, when I am creating a thumbnail, the built-in images appear, but the asset library images don't correctly appear. I know the imageView is there because elsewhere I set its background color to light gray but the image itself is absent.
I suspect that there may be an asynchronous element to loading the images into image views, and the image doesn't get there fast enough when its an asset library image being rendered into an ImageContext.
Is this possible? If not, why would renderInContext work fine in a PDFContext but not so well in an ImageContext?
And more to the point, how can I get my asset library images included in my thumbnails.
Both the pdf representation and the thumbnail are created via methods in scrapbookView. The code is as follows:
To create pdfs:
-(NSData*) pdfRepresentation
{
//== create a pdf context
NSMutableData* result = [[NSMutableData alloc] init];
UIGraphicsBeginPDFContextToData(result, self.bounds, nil);
UIGraphicsBeginPDFPage();
//== draw this view into it
[self.layer renderInContext: UIGraphicsGetCurrentContext()]; // works for asset library images
//== add a rectangular frame also
CGContextRef pdf = UIGraphicsGetCurrentContext();
CGContextAddRect(pdf, self.bounds);
[[UIColor lightGrayColor] setStroke];
CGContextStrokePath(pdf);
//== clean up and return the results
UIGraphicsEndPDFContext();
return [result copy];
}
To create the thumbnail:
-(UIImage*) thumbnailRepresentation
{
//== create a large bitmapped image context
CGFloat minSpan = MIN(self.bounds.size.height, self.bounds.size.width);
CGSize imageSize = CGSizeMake(minSpan, minSpan);
UIGraphicsBeginImageContextWithOptions(imageSize, YES, 0.0);
//== draw the current view into the bitmap context
self.backgroundColor = [UIColor whiteColor];
[self.layer renderInContext: UIGraphicsGetCurrentContext()]; // doesn't work for asset library images
self.backgroundColor = [UIColor clearColor];
//== get the preliminary results
UIImage* image = UIGraphicsGetImageFromCurrentImageContext();
UIGraphicsEndImageContext();
//== create a smaller image from this image
CGFloat thumbnailWidth = 40.0;
CGFloat scaleFactor = thumbnailWidth / minSpan;
UIImage* result = [UIImage imageWithCGImage:[image CGImage] scale:scaleFactor orientation:UIImageOrientationUp];
return result;
}
// btw - seems wasteful to create such a large intermediate image
Any help would be appreciated.