If you see a random head on google image search it's not necessarily a free head, it might be leaked, but still belong to someone else.
If you see a free art on toribash forums in one of those "free to use art" thread, you can wear it for free on your tori, however you are not allowed to try and profit from it in any form. That means you are not allowed to sell it or alter the image and then claim it as your own.