Can you post the VI you are running that performs this action? I don't know what you mean when you say you use an Invoke Node, are you using the FP.Get Image method? Also, could you better describe what you are trying to do? That would help me to point you in the right direction.
I've been playing around with you VI a lot and I think the problem is that the ActiveX Container class Invoke Node just isn't meant to be used to capture video screen shots. The method options of the Invoke Node don't change depending on what is inside the container, so I would imagine that the Get Image method is designed for the Picture Box and possibly other controls but not for video capture. However in my searching for answers I did stumble across a free image toolkit that uses different methods to capture the front panel screen. I can't guarantee it will work for your application since it isn't an official NI supported product, but you can download it from the website below and try it out to see if it provides what you are looking for.
The only way I can capture the image of WMP is by actually using the print screen function, and even then it's the whole screen not just the video. You can make calls into the windows .DLLs to do this programmatically but that is a rather intensive thing to try. There are a few examples of this on www.ni.com/examples/ if you search for "keyboard functions" that you could adapt to cause a print screen, however I'm not 100% confident that even that will work.
