How can I add an image to a pool?

I've found this image https://danbooru.donmai.us/posts/4670367?q=gawain_%28fairy_knight%29_%28fate%29 and I'm contemplating in placing it on a pool. Any idea how to do it?

You can use the pools page or type pool:pool_name_here into the tag box or click "Add to pool" in the sidebar.

You can also use the pool ID and type pool:ID (like pool:12345) into the tag box.