particles.setTexture

particles.setTexture sets Texture on a particles object. It takes id and url.

particles.setTexture(id, url)

function start() {  particles.create('main', 10);  particles.setTexture('main', '/textures/spark.png');}

particles.create is there to give the call something to act on; particles.setTexture is the line that matters. The code sits in start(), which the engine calls once, when the entity spawns.