I can’t reproduce this, so currently wondering if you do really have all your media objects with the correct “type” setting.
Not sure what you are defining as “not very interesting when presented as small images in the Slide Show” but I would guess that images of census pages might fit that definition. You have 52 such images on your site designated as “Type: Photo”
To check, go to Lists > Media objects, select “Photo” in the “Type drop-down. To filter to just census images, enter “Census” in the Search filters box.