The solution was with the Resources folder. I had imported the snow.png and it had been placed in the wrong place.
The code as written is looking for the file/files in the Resources/Images/ folder structure. When I created this structure and reimported, (instead of just Resources folder structure (no image subfolder)) everything works correctly. Thanks UseCase for pointing me in the right direction.
Another question along these lines, what's the best way of specifying a different particle image?
The easiest would be to just switch out the image and keep the naming structure the same.
I also tried something like this:
ps.texture = [[[TextureMgr sharedTextureMgr] addImage: @"fire.png"]retain];
It works. Just wondering if there's a different way of doing it.
Demetrius