Captured Image (Reduced Quality?)

[Moganium]

I've read through several posts regarding capturing a frame, but somehow the images that I get always seem to be a bit degraded from what I see on the screen. I think it's because the image is picked out from the source rather than after video rendering, so I was wondering if there's anything to get a frame exactly from "what you see on the screen", kind of like print screen. Also, I tried capturing frame with original size and "what you see" and they both turn out to be free, so what's the difference in that, does it not go any bigger than the original size?