Using thumbnails

When uploading a picture, we're given the option of generating a thumbnail. I want to include a thumbnail in my story so that when the user clicks on it, the normal-sized picture is uploaded, which is the typical use for a thumbnail..

I've been messing with html and can do it if I put a link to the main picture from a title beneath the thumbnail. But I'm not certain how to link the thumbnail itself to the main picture. (It's worth saying I haven't much used html for 15 years, so I'm quite rusty!)

Is there an easy way to use thumbnails on this site?