I think the post office went with drawings because they just look better in smaller format than photos. A reduced photo would look kind of amatuerish. An artist's rendering of the tree looks like artwork, rather than an attempt to reduce a photo to cram it on a stamp...It can also be painted to fit the format. Just look to the postage stamp treatments of Elvis, baseball players, wildlife and other "specialty" stamps by the USPS. They are all artists renderings for the most part.
And species don't really make much difference here. People don't really understand what "bonsai" is, much less "native bonsai."