I investigated this pretty thoroughly. What I did was to import wowcombatlog.txt into Access and look at actor guid's, not WWS.
- Shadow images spawn in threes.
- They come in one of two forms: Shadowfury or Dark Strike. You can get unlucky and get 3x SF, lucky 3x DS, or a mix.
- Dark Strike images can change targets but it's rare (they appear to do a vanish animation first).
- Shadowfury images die after casting once. They take 0.5 seconds to cast Shadowfury.
- Shadow Images have an approx. 10 yard deadzone on target acquisition.
They ignore this deadzone if they can't find a valid target outside of it.
- Shadow Images acquire targets based on a random selection within LOS of Sacrolash.
I believe no two images can pick the same target, though. (edit: refuted below, ignore)
- Shadow Images survive approx. 10 seconds.
We spent two weeks trying to copy videos until we sat down, figured this out, and eventually adopted a strat that kills Sacrolash first. Having tried and failed horribly at the alternative, i'm absolutely convinced that killing Sacrolash first is far easier.
One of our attempts involved stacking the entire raid on top of Sacrolash and putting three people in SR on the balcony. They reliably pulled every image until they died (which admittedly was pretty fast).
Edit: I just noticed i'm posting directly after my own guildmate who basically says the same thing.