@Vincent: I thought about adding the properties one at a time but it seemed a bit like overkill for the tiff encoder, which has only two options. But I will defenitly do this for the jpeg encoder, which has much more options.
Well, actually, on further consideration I can understand why you did it this way, since adding a property is very easy and doesn't really require implementing that property. Though it'd be nice to get some notification when a property is used that we don't support.