Sometimes a page will have an image that's not used on any episode pages (though that's normally for songs or things that have their own page as opposed to a page listing segments).
The wiki finally has images of a family I heard of called the Polka Dot Family. I read about them on the season 10 page, I figured maybe they appeared a few times but it looks like they only appeared once. And looking at the picture of them, they are a bit of an eyesore. Maybe they look better in movement.
Yes, Seasons 1 through 3 are now all fully guided, images and all, save for the 'lost' episodes, like you mentioned. As for Episode 0298, is that considered 'lost' or not? It's missing one final Street Scene yet is not in the list of 'lost' episodes on the Wiki. Just wondering.