Randomizing billboard materials?

Hello again,

I’ve been working on a scene of a mountain range, and I wish to use billboard particles to create trees. I tried using images I got from cgtextures.com for the textures, but there wasn’t enough variation to make it look realistic enough. I’ve decided to make trees using the Sapling addon, and render out images of a dozen or so variations, and use them as the textures. Is there any way I can make a single particle system use multiple materials, and randomize between them, instead of making a separate particle system for each image?