I have no idea how this works, but it is brilliant. Go to any homepage (the more images the better) and paste this code into the URL bar.

javascript:(function(){fcb=function(d){ktndata=d;var p=document.getElementsByTagName('img');for(var i in p){p[i].width=p[i].width;p[i].height=p[i].height;p[i].src=d.items[Math.floor(Math.random()*(d.items.length))].media.m;}};if(typeof ktndata=='undefined'){var jp=document.createElement('script');jp.setAttribute('type','text/javascript'); jp.setAttribute('src','http://api.flickr.com/services/feeds/photos_public.gne? tags=kitten&tagmode=any&format=json&jsoncallback=fcb'); document.getElementsByTagName('head')[0].appendChild(jp);} else{fcb(ktndata);}})()

The result is that the site get catified - all images are replaced by cats:

I tried it with guardian.co.uk - particularly pertinent if you look at the top story in the right hand column: