I was wondering where I would post a Clan's art thread.

You cant obviously put it in the Clan Discussion
You cant put it it the Items section
You cant put it in the rapid threads section

What do you put in the art section? Yes art.
What do you put in the Art/Textures section below market? Yes art you sell.

You could use the art section of the forum and the other art to sell. If you are an official clan you could put your art for sharing on your clans personal section. You could just share it on your DSC for now.Your choice.