One of the features in the Prism Webapp Bundle for Google Wave is a toaster pop-up notification of unread waves using the window.platform.showNotification() method. The third parameter is named aImageURI, and is described by the nsIPlatformGlue IDL as, “The URI of an image to use in alert. Can be null for no image.”
Which is great, except … what URI scheme and path does one use? Every example I could find always passed null for the image, so after giving up on the web I joined the Prism mailing list and posted a question. The first response was to use the inline data scheme, with a base64-encoded image. It was ugly, but it worked.
A later response, however, yielded the proper way: