It gives you a 404 if the image doesn't exist, so you can use that. It's all in JS, anyway, so the gosquared server does't need to know any of this.

We've got it so that the image 404's and we listen to the image error event to fade the G in or out.

You can specify a default image URI, and possibly look at the pixels of the returned image to see if it is the specified default.