 So you want to change the particle sprite with something custom, not a problem. You'll need to right click and create a new material, then open it and set it to quad. Usually you'll want to send blend mode to translucent and right under set it to unlit. Now if you have a transparent PNG, hold T and left click to bring up a texture node. Then set it to your image and drag red into emissive and A into opacity. Now if you have a black and white image though, right click, set it to particle color, hold T and left click to bring up a texture node. Then hold M and left click to bring up a mix node, lit, and set it to the correct image. Then drag RGB to emissive, A to A, R to B, and mix to opacity. Save, open your particle, and at the bottom under renderer, material, set it to the material you just created. You're done. Hope that helps and as always, hope you have a fantastic day and I'll see you around.