bullet.png, energy.png, scorch.png, blob.png
    Public domain sources

smoke.png, and explosion.png
    Created by RaZgRiZ Made with Filter Forge II Beta 3
    Licensed publicly for everyone to use, modify and distribute as long as the author is credited for the original work.

entity.png, lightning.png, hint.png, mflare.png, sflare.png, spark.png
    Created by Quinton Reeves, CC-BY-SA 3.0 license, or any later version.

blood.png, fire.png
    Created by FischKopF, source: http://www.quadropolis.us/node/2693
    Uploaded explicitly as "Public Domain", with the additional comment:
    "Feel free to edit, claim it's yours, use it in own projects"

muzzle.png, plasma.png, spark.png
    Created by Luckystrike-Rx

snow.png, stain.png
    Created by Hirato for Sandbox