Go to the Test Center tab. Scroll down the page to the 'Bookmarklet' section and find the 'Drag Me' button. You can either drag it into your favorites list or right click on it (on Mac: control-click). Here is an article that discusses the techniques in the various browsers - http://www.oreillynet.com/onlamp/blog/2005/09/writing_instructions_for_insta.html.

Versions of Google Chrome will not allow you to drag-and-drop the image. Read instructions for the work around here: How do I install the bookmarklet in Google Chrome?