Make a .zip folder named Decal_something, replacing something with the decal name of course.
Create a square image, I find 256x to be the best, but bigger still works, and place that inside the .zip folder. This will be the actual decal.
Create a copy of the image and resize it to be 64x, then create a folder named thumbs (no capitals), and place the smaller image into the thumbs folder then place the thumbs folder into the .zip folder.
You can add a description inside if you want people to know who made it, but you don't have to.
If you want other people to be able to download your decal, upload it to a file sharing site like MediaFire, and just place the download link in a post in the Faces, Decals, Prints section of the forums.