Sorry for my poor artwork - we learn by doing, and all that. Mind you, I have done (or attempted) some fancy shading on the box to imply depth.
I had the idea for this last year, and I'm quite surprised that nobody has beaten me to it (or at least I couldn't find any prior art through a Google search). It was too late for cards last year, so I put it to one side; my plan was to draw it this year, then maybe make some custom cards at a printing shop. However, I didn't get round to that in time, so I figured that I ought to at least do something rather than putting it off for another year.
I do have another idea for a card, but you'll have to wait until next December to see that...
I was wondering about signing this somehow - I've seen lots of comic strip artists doing that (e.g. Pete Abrams). However, I wouldn't be able to write legibly with my drawing tablet. A better option would be metadata, but I can't work out how to transfer the information from Photoshop Elements to the gif file. Any ideas? I'm guessing that photographers have already dealt with this issue. Still, I don't think I have to worry too much about people stealing my drawings without giving me credit :)