If you open the ".rul" file, you will see a section called "extraSprites"at the bottom. In there are the path to be used to access the new graphics.
Just make sure that those lines in the .rul file match with where you put the files (also the names, the ones in the ruleset are for red plasma, if I remember well).
In general, it is considered best practice to put new files in the Resources folder, within a folder with the name of your mod. It is not necessary, but that's how every modder does it (and how the .rul file for this mod assumes you will do things as well, but you can change it).