So part of David's suggestion was key apparently:
DarkGDK.Engine.InitializeGDK();
I wasn't doing that at all. Now that I initialize the Engine, I no longer get the NullReferenceException. Apparently that wasn't telling me it couldn't find the file after all. However, it's doing something I didn't expect. Instead of pasting the image into my form, it's opening a new form window and pasting the image in there.
Maybe if I step back and explain what I'm trying to do. I've got an existing C# Windows form application. It's simple 2D, top-down, turn based game. I'm building up a large map from tiles into a scollable panel, and I have another panel with buttons and other controls. I'm using the built in System.Drawing functions now and everything works. It just draws kind of slow of course. So I'm trying to replace the System.Drawing methods with something faster. Looked at DriectX but that's a bit overwhelming.
I've looked at the documentation and some of the tutorials and not really found anything directly applicable to what I'm trying to do. I was hoping to just learn enough to make soem simple changes to my code and not have to rewrite the entire application.
If someone could point me to a good tutorial or some example C# code it would be appreciated.