Ok, I was going crazy with this addon too, and it just wouldn't work, despite my having followed exactly all the instructions in this thread.
It works now, however, after I uploaded the version that se.nsuo.us provided. I was getting tripped up because it said "Keeping the fullsize image too? YES!". But now it says "Keeping the fullsize image too? NO!" and it DOES NOW keep the fullsize image.
Is that an incorrect label or am I just in opposite-land?