Hmm, maybe Photofonts would work. It's a new font technology that allows for full-colored bitmap fonts. The Photoshop plugin is free. Creation of Photofonts is free and simple, if you know how to program an XML file. If not, there's a program you could buy to help compile your images into fonts.

The reason I think Photofonts would work well is because inserting your images as fonts would allow for fast and easy rescaling. Also, you could easily save many tree images as one "forest font", for example, and then you could have lots of variance in the forests in your map, WITHOUT the tedious copying of zillions of layers. Might be worth checking out at least. Do a google search.*