Trouble! Taking a portion out of an image.

Question

Ok, I HAVE looked around, and I can't seem to find a way that doesn't rely heavily on Directx generation, what I want to do is take an image, a fairly large one, say 1000x1000, and take a portion of that out. Say, 500x500. I do NOT want to scale the image, only to "grab" a portion of it. I've searched through commands for half a day now, and I can't find a way, they all change the SIZE, and I can't seem to find what I need to get a portion of an image. I know you guys are really good at this, but I'm just a student (first year C#, but I have programmed in other languages), and I've never needed to grab just a portion before in C#. Am I overlooking something obvious? Please help!