Quick: Can you spot the problem with these three lines of code?
BitmapImage bi = new BitmapImage();
TheImage.Source = bi;
These statements create an image from a stream of PNG or JPG image bits and display the image by assigning it to a XAML Image object named TheImage. It’s boilerplate code used to display images read from the local file system or obtained from a service. And while there’s nothing inherently wrong with the code itself, you’ll want to think carefully before including it in any Silverlight application.
I call it “Silverlight’s Big Image Problem.” Not the kind of image problem a movie star might suffer, but an inherent memory-consumption problem when dealing with large bitmap images in Silverlight.
The problem manifests itself when you handle large images in large numbers. The My Pictures Viewer that I blogged about yesterday is a case in point. When a user running the application selects a folder containing one or more image files, the viewer displays clickable thumbnail versions of the images. The problem is that because Silverlight’s BitmapImage class consumes massive amounts of memory (up to 40 or 50 MB per image for a typical 2 to 3 MB digital photo), you simply can’t have too many instances extant at once. But to create a thumbnail, you first need a BitmapImage that wraps the entire image. You might create a thumbnail by assigning the BitmapImage to an Image object that measures just 100 by 100 pixels, but if the original image measures 4,000 by 4,000 pixels, it’s the latter figure you pay the price for.
from Jeff Prosise more here