Billing via log files, surely?
Surely they can do some kind of tiered billing by processing the logfiles to see who the big users are and then bill accordingly? Maybe even keep it free for the small-scale users, and the ones using it to host popular hi-res stuff probably knew this day was coming anyway. The suggestion earlier for 'click to view' would make sense, even if accompanied by a (preferably non-rage-inducing) ad banner.
I'm tempted to blame the advertisers, for making their ads so effing obnoxious and stalkingly pervasive that a sufficiently large number of people blocked them.