By blank images, you mean just a white png?
Not white, you want the images to be fully transparent. So nothing on them at all. E.g. if you're using GIMP, you should only see the background canvas grid and nothing else.
One simple way to do it would just be to go through the dirt images and reduce the opacity on all of them to zero. That way you wouldn't need to mess around with filenames. I also don't know if the canvas size has to match the originals to avoid problems (and the canvas sizes vary frequently), so again, just reducing the opacity to zero would bypass that potential issue.
Also, to save yourself some trouble down the line, make sure you keep a separate backup of your edited blank images with all the correct filenames. Because every time you install a new game version you'll have to replace the original files again.
