Why does it not position itself in the top left corner and cover the entire screen?
Origin should be top left from my point of view, and Screen.width, Screen.height should cover the screen. What have I done insanely wrong?
var Background : Texture2D is loaded with the image of that planet. MainMenuSkin.box has had it's standard Unity textures deleted because I do not want to use them. It has worked perfectly to do that in the past.
Use GUI.DrawTexture instead of GUI.Box
The Box itself is most likely covering the entire screen, but the positioning of the GUIContent (in your example, this image) in your GUI.Box depends on what the settings are in your GuiSkin.
Is the texture large enough?
Look up in the Free Aspect and place the last option, which is what you choose for your game, but when you build an executable will run as follows: Open the executable, select the default resolution and window mode when the game open, press ALT + ENTER ...
Hope this helps