Comment 4 for bug 1324142

It looks like all that's really needed on the scopes side is a documentation update. I've documented a "art-fallback" in the "components" dictionary of the category renderer. That allows a scope to specify a category-specific fallback image. I think this should be enough of a hook, but the shell will need to actually use the field if it cannot retrieve the artwork for a particular result. If no fallback image is specified, or the URI for the fallback image cannot be resolved, I guess the shell should display some universal placeholder image, so there is at least something on screen, instead of nothing at all.