Is it possible to make the custom wallpaper background image STRETCH to fill the background instead of TILE to repeat it?

Nope. The only thing you can do is upload a really large background image, which isn't really recommended because on some larger screens it still may tile anyway, and load time can be significant if you're uploading a gigantic photo background image or something like that. Many backgrounds seamlessly tile fortunately, and they're very small and quick to load. Just Google "seamless textures" or "seamless backgrounds" and you'll find all kinds of things you can use that will look great at all resolutions.