Upload & Sell: On
In some cases, the person who originally posted an image might removes it, to avoid future problems. I've done it. For example, I'll sometimes post an example image that I think is relevant to a thread, but that image is from an event that I normally wouldn't share photos from, and especially when the image includes people and there is no permission that covers publishing their photo on the open internet. Generally, I'll delete the photo after a couple of days; sometimes, after a few hours.
Images are "stolen" from all online sites, all of the time. That's another reason that original posters delete some of their images after a short while, especially if they're unique or special in some way.
Of course, there's also some cases where an image link in a FM thread is to a server or hosting site that no longer has the image, or isn't online, or whatever.
I don't think you're missing anything, except the original photos that have been deleted, or downsized.