Keep in mind that unless it has a specific purpose, no one is going to put a truly ginormous image on the web (other than the periodic anomolies we have discussed above that weren't "web prepped").

The ONLY reason I have actually worked with truly enormous images is because I worked in a pro-lab where we did 6' wide images (the rolls we used were 72" wide) to any length the client wanted/needed - 14', 50', etc (and even those tended to be 200dpi max given that NO ONE was going to be within 5 feet of them to look up close to check for any pixellation - many roaodside billboards are 12dpi - and 36dpi tops!). And these were saved onto portable HDs and brought to us to print out of a Lambda.

Most DSLRs top out at 12-ish MP, so there isn't a plethora of large images beyond that taken today. Every once in a while someone will need a huge scan of a medium or large format neg/transparency, but again, that's use specific - if that huge scan was prepped to go on the web it would be downsized to something viewable without having to be tiled - typically so people won't have to scroll side to side, or up and down, though some are left a little bigger.

Hope this makes sense-
-Lew