How Can I Capture Image From Silverlight Application Within Web Browser
Though this is a very old question but I came across similar problem before and want to share the details here.
When you take screenshot using selenium library it calls web driver method to translate the whole page content into an image. But for the silverlight component its not able to capture it for obvious reasons.
Solution is that you need to take current screen image rather than relying on browser. One of the easiest way is to use Robot Framework SikuliLibrary. It also have Capture Region keyword which can take snapshot of specific part of the screen. I hope this would be sufficient for your requirement.