Hi Andrew!
The default vBulletin setting is for thumbnails, for the reasons you've stated.
As far as I know, the setting is not switchable by individual users.
If and when Andreas decides to save money and bandwidth by going away from the full-size images which have been the tradition on the Forum your wish will come true.
Perhaps a PM to Andreas would get a more authoritative answer than I can give. He is the grand wizard of the electrons around here.
Thanks.
Allan