I think you can post it from this Marketplace (Altcoins) Section or you can post it also in Goods section.

I don't think you can post this art in collectible because mostly post there are related to Bitcoin physical coins.

However, I found this post selling art on the collectible section from here https://bitcointalk.org/index.php?topic=2700386.0

It seems that you can post this art from the collectible section just provide the image of the art.