You'll probably have to sudo cp them. If you don't know how, I would
STRONGLY recommend learning a bit about
Unix and the command line before attempting something like this.
I could
probably walk you through it if you gave me the image file names, and locations on your computer (that is, the ones that you want to replace the trash icon with). (And to save as .png, open the images in Preview, and then click File>Save As..., and choose png).
Good luck!