If you save it as "script.user.js" (.user.js is important, apparently) and drag it into the scriptmonkey management dialog, it should add it. I don't know if that works in your interface (Windows here). If it doesn't, select "new user script", fill in anything for the field names and "*" for "includes", and then paste the script into the file it creates (it loads an editor for me).

If it's loaded, when you click the greasemonkey icon (in the bottom-right) you should see the script name with a check next to it.

Caveat: the image needs to be part of a webpage, and not loaded directly (greasemonkey doesn't let you run scripts on anything but HTML).