If you rename an image, it must have the name of an image in the images folder, including the path relative to the image folder.
You can't have two images with the same name in the same slot. Image names are unique per slot. The image name uniquely identifies an image.
Again, when I say image name I mean the path relative to the image folder and the file name. Eg, you can have "goblin/head.png" and "goblin-girl/head.png" attached to the same slot.