You are not doing anything wrong, the code on the site for these things is currently broken. This is a consequence of some dramatic overhaul of all the image data, which did break quite a few things (most is easy to fix, this one took about 10 minutes to figure out what's wrong and how to fix it, but you have to know it's broken first).
I have provided a fix to Jools, which he will review and upload (assuming it's "ok").