(See the best response by FKI.)
If I wanted to add a .PNG picture over my game screen to represent the GUI, how would I go about doing so?

Best response
You would create an object type to be used as HUD object added to client.screen.
I came up with this:
icon = 'Game Outline.PNG'
layer = MOB_LAYER + 5

new /obj/gui_outline(src)

My screen seems to be zoomed in even though in the interface I set icon size to 32.
In response to Viper
Did you ever change the map's zoom setting at any time? (Not icon_size, but setting 'zoom' using winset().) If so, you're going to have to set it back to 1 (or whatever you want) since it persists through sessions by default.

If this isn't the issue, a screenshot of what you're seeing might help.
In response to FKI
Haven't used a zoom setting. This is what I get though. When I disable the skin, everything looks normal.
For the sake of testing, what if you were to create a new, barebones skin with similar settings? I'm curious what you would see then.
In response to FKI
Seems to be the same outcome. I am quite puzzled by it. When I deactivate the skin, it goes back to normal view.

Edit: Actually, I fixed it. I had to use Size to View in the Map section of the skin. I set it to 25x19 and it now fits properly. Thanks for the assistance!