well to insert ow's in a fire red rom or leaf green or ruby saphire or emerald u need tools like:
overworld changer rebirth edition or
nse(nameless sprite editor)
there is another tool but all say it's really buggy.
as for rpgxpmaker i don;t have a clue how to insert ow;s i never used it.
It does have to approved, yes, and you can scroll down and read The Drawing Board section of the rules thread right there.
Also, if you scroll down to the bottom of the Game Dev area (or any area of PC), it shows the usernames of two moderators that watch it. Hope that helps.
oh well itrs not mine, any quetions you can ask on that thread me, i dont know how to insert tiles. i might be able to insert the tree, but with the fr/lf tree pallete