Adding Custom Tiles
By clicking the "+" button found at the far end of the Terrain and Foreground menus, you get a submenu that opens between them called either "New Terrain" or "New Foreground" respectively.
Simply paste a url in the input box. This will generate a small preview of how the new sprite will appear. Then click save.
This will then add the sprite to the correct menu. You can then use it as though it were any other tile.