Boids: how do I use a random sequence?

Artwork Blender Tests
1

I put together a test where I have boid particles follow a terrain surface. For the particles, I used an object which is a plane with an image sequence as the texture. Here is the result:

http://youtube.com/watch?v=15T41XZBvys

The question is, how do I randomize the image sequence for each particle? Right now all the horses gallop in unison. What I want to happen is that the image sequence starts randomly for each particle.

Thanks,

Terry