Image annotations could be oa:SpecificResource

http://iiif.io/api/presentation/2.1/#image-resources suggests that the annotation resource '**SHOULD** have an @type of “dctypes:Image”'

One reasonable exception to this is where the image annotations are of @type `oa:SpecificResource` - e.g., to crop out scanner machinery. The spec should warn that some clients might not be able to cope with this (it has to handle the crop).
