There's a great easter egg (or should that be christmas egg) in the Flickr note script... This morning I was quite surprised to see that a santa hat had suddenly appeared in one of my photos! It turns out that if you add a note with the text ho ho ho hat to a photo, a santa hat will appear. You can even stretch it to proportion. Similarly, a note with ho ho ho beard will make a big gray santa beard appear.

(I checked the Flickr notes.js to see if there were any other special codes, but these two seem to be it.)