From rob on IRC, origin unknown, try this:
Go to a web page with lots of images, e.g. news.bbc.co.uk
Paste the following text into your address bar and watch the magic:
javascript:R=0; x1=.1; y1=.05; x2=.25; y2=.24; x3=1.6; y3=.24; x4=300; y4=200; x5=300;y5=200;DI=document.images; DIL=DI.length; function A(){for(i=0; i