Summing it up:
1. Add an interface
2. Add a map control to a window
3. Uncheck default, and name it "map2"
4. Try this code
mob
verb/loadcharacter()
var/file = input("Which character sheet would you like to use?", "Load Character Sheet") as null | icon
if(!file)
return
var/atom/movable/atom = new
atom.screen_loc = "map2:1,1"
var/image/i = image(icon(file), atom)
if(!i)
alert(usr, "Could not create an image! Check file header", "Error", "Ok")
return
src.client.screen += atom
src.client.images += i
Nothing will happen. Now add another map control and set it to default, and the code will work. Not sure if this is intended behavior or not but its certainly not explained in the documentation. It mentions nothing of having to have a default map control in order to use this type of screen_loc positioning.