Ugh /facepalm
You make a folder with the normal size decal in it.
You make another folder, called "Thumbs"
You copy it into Thumbs, then resize it to 64x pixels, with the same name.
You then go into the original folder, select the original image, and Thumbs, make it into a .zip of any name you feel like.
I don't recommend doing so, however, because this is a pretty low-grade decal.