How to submit tilesets and sprites

Hello,
I have used tilesets from this site in the past.
I am using tilesets from Star Wars Battleground ripped a long time ago.
I have also a lot of tileset that I have made.
How do I post them on this site? How do you submit? Do you need to post them elsewhere?
What format do they need to be in? What are the steps? Help!