How to post to a thread without a New Thread button

Heeeeeeelp. Yesterday I wanted to post a photo to the Loch Of the Lowes osprey thread, but no matter how I tried, I couldn't find the New Thread button.

How to I add my pics to such a thread, its driving me demented

Remember - the only stupid question is the one you don't ask.

See some of my photos at  https://www.flickr.com/photos/181900595@N03/