Note: It won't be masked when it returns, you have to use it with the magic Javascript for that. This is just a bare image coming back.
Also note: This web server is running Lighttpd and will fail if you use curl to drive it unless you supress the "Expect:" header with a -H "Expect: ".
As long as we are noting: I might retain a copy of your image for debugging and tuning, and this is just a tiny slice of a VPS, please don't kill it. The source is available at http://jim.studt.net/jpeg-alpha/ if you need to do a large number of files.