Why do the icons on my internet favorites bookmarks eventually dissappear?

Hey guys,
This is something that has always bugged me. Eventually, all those little icons to the left of the bookmarks dissappear? Why is that? Large groups of them tend to all vanish at the same time I have noticed? I use Internet Explorer 8 Beta, but it's happened on every single version or IE I've used in the past 8 years that I've seen.

Is there anyway to get them all back easily without going to each site one-by-one and adding the favorites again?

I believe they are stored as temp images. Something like a disk cleanup, delete temp files, and the like would most likely remove them.

Also when you clean out your cache, they tend to disappear until you revisit that site again

Hmmmm, they never come back once they dissappear? The only way I can get them back is by revisiting the site, then adding it back into my favorites again... Its not the end of the world, just annoying.