You need to make particle Decals using a program that can insert a see-through type of background, like Paint.net
Size needs to have square dimension and be a multiple of 4, like 4x4, 16x16, 32x32, 64x64, etc. This does NOT affect the size of the particle in-game, it only adds more detail to particles as you go higher.
You're going to need to use 2 datablocks to insert the particles in-game, the particleData and particleEmitter datablocks. You can look into the basic particle emitters that come with the game, found in the Particle_basic.zip file.
