The difference being that pictures are commonly inserted inline in the web for discussion/humour/other purposes more than video. When a video is inserted in a page, more often than not it's the focus of the page.

I always thought of rehosting images as the courteous thing to do. Say someone made an image macro that you want to insert inline into a post on a board somewhere. Depending on the popularity of the forum, hotlinking it could suck up their bandwidth allotment and get their site knocked offline. The nice thing to do would be then to rehost it (using Imgur or the like) and link it that way.

Well, Facebook not only takes parts of text, but also displays thumbnails of actual image content of the linked site.