press the print screen key (usually next to F12)
open a graphics program (paint, ps, psp, irfanview, etc)
edit > paste it in
save it
upload it to a hosting site like photobucket (it's free)
copy the http address the site generates
paste that into the box that pops up when you click the 'insert image' icon (6 buttons to the right of the 'insert smiley' icon here