For link recommendations, we used the "no suggestions found" error dialog for errors that are in theory transient (although in practice it didn't work so well): the search index is temporarily lagged so invalid tasks show up. Those tasks would disappear eventually as the index catches up. For image recommendations though, we show the error for some cases that the search index just (currently) cannot be aware of: invalid image type or size, the image having been deleted, or the article already having an image. We should avoid these erroneous tasks showing up forever in people's task queues.

One approach here would be some kind of maintenance script. The other is to remove a task after we detect we can't display it to the user (either in HomepageHooks::onBeforePageDisplay(), where it's easy to follow up with a removal, or in AddImageArticleTarget.afterSurfaceReady() where it's not so easy).