I gave myself a crash course this weekend. I mainly wanted to be able to make plugins for fun in Pixelmator. But turns out you can use things in iChat and Photobooth live. It was a bit of a fun uphill battle to actually figure out a repeatable process. So I wrote one. You can download my Image Units Tutorial in PDF.
I cover Prototyping in Quartz Composer, moving it to an Image Unit and compile it in Xcode. I toss in how to add a user input and even found a blog post on the Internet on how to ensure your IU puts out an image with defined dimensions.