1. Make a .zip folder named Face_X where X is the name of the decal
2. Do what Regulith said, "Create an image that is a square with sides a power of 2. 256 is generally good. Make a 64x64 copy of it."
3. Create a folder entitled Thumbs
4. Place the larger image(s) (Not the 64x64 one(s)) inside the .zip folder
5. Place the 64x64 image inside the Thumbs folder, and then place the Thumbs folder inside the .zip folder
The 64x64 image will act as a thumbnail image of the lager image, so make sure they are named the same thing. Also, when packaging with Mac, it adds a bunch of files that will screw it up, so send it to someone with something other than Mac and knows how to package decals so they can get rid of all the unneeded files, like me. Making faces is the same as making decals only you name the .zip Face_X where X is the name of the face.