i am happy to see that it reached you. i thought it might have been pushing things too far. looks like they turned it upside down for the cancel. i did not take time to reproduce the gorey drawing. i made a copy-paste-print image and then altered it a bit. it's perfectly legal to appropriate images as long as you are not selling them.
I think the sorting machines sense whether there is a stamp in the corner of an envelope, then flip the envelope so that the stamp is top right for franking. You can see how they don't love unusually placed stamps as they have to be looked at separately, and slow the process. Those machines are pretty amazing, though.