Put them in your modpath
Always make sure your .png files have the EXACT same name as the file your are attempting to overwrite. There should be no problem if you are sure to do that step correctly.
These are for Fire2Final Fantasy VII1\mods\none\magic\faira00_00.pngFinal Fantasy VII1\mods\none\magic\faira01_00.png
in ff7_opengl.cfg enable show missing textures, start the game, go into battle, use magic that you need, than check app.log for the path and the name