TESTING pikachu school/office folder for mac
open up the png pikachu file and click edit select all
all you have to do is right click on the folder image u want to change and click get info. then unlock it (bottom right) click on the very top image so that its highlighted (blue) then paste the image lock it up and there you go! if it doesn't work let me know in the comments! as i said before its just a trial!
there will probably be more to come on my other account