It’s a reference to the third episode of Monty Python’s Flying Circus, which was entitled “How to Recognize Different Types of Trees from Quite a Long Way Away.”
I can’t use it until I spring for a paid account [Edit: now I have one!], but maybe someone else will enjoy it in the meanwhile. I’d appreciate being credited if you use it, but no biggie.
(I did have it throwing in a “No. 3” slide occasionally, but doing so made the file size much too big for use as an LJ icon.)