EDIT: On the issue of this possibly being "modding" thread, I was given permission to post this by one of the staff before I created it.
I ran a quick search on one of the registry entries required for this trick, so I'm assuming this hasn't been discussed here before. Another member told me about this last night, so I ended up trying it for myself, and the results were pretty spectacular.
Apparently, setting the width and height to your monitor's default resolution causes this to not work correctly. I just received a new 1440p monitor yesterday however, and was able to get the game running at 1920x1080. The maps that I took the screenshots in seem to be relatively unaffected by higher resolutions. This isn't the case for all maps; Henesys, Lith Harbor, and Kerning City all had broken backgrounds. But for the maps that it does work in, its a beautiful thing. It makes me wish that they could rework the entire game to work the way that Ellinia, Sleepywood, and the Free Market did because it was a major improvement when it worked.
UPDATE: The maximum resolution that Maplestory can support is currently 2048*1536. If one of your resolution parameters (either your game width or game height) exceeds this, this method will either glitch or default back to 800*600. To avoid that, be sure to set the game window's width to either 2048 or below, and the games height to 1536 or below.
UPDATE 2: Reported to no longer work. See page 2 of this thread for more details.