I'm pretty sure those tiles can be found in the 'Pokemon World Online' folder on your computer ('C:\Program Files\Pokemon World Online\Skins\Default\Tiles' on default), I have no experience with Tiled, but that's where you find the actual tile pictures.
The Tiles used by Tiled should exist in a folder called "tiles" within the program's main folder. To the best of my knowledge, while Tiled uses a different required size for each image (800x1024px instead of PWO's 256x1024), the Tiles are otherwise the same as those used in PWO, and the additional space is unused.
The PWO version of Tiled already comes with the tiles included. Note that unmodified Tiled will not be able to save maps in a compatible format for PWO, so they can't be used for contest entries.