If you're making your own:
1. Make a 256x256 decal, save to desktop.
2. Resize to 64x64, make folder called "thumbs" on desktop, save to folder.
3. Make .zip file. Name "Decal_X", replacing X with what you want to call it.
4. Put large decal and thumbs folder into .zip file.
5. Move/Copy to Add-ons.