Make sure you have a thumbs folder with a 64x64 version of the image.
You probably won't want to go through the trouble to do this for each individual decal, so you should start releasing your decals in packs.Create your decals. Create thumbnails for every decal, and put them inside of a folder called thumbs.Select all of your decals/faces and your thumbs folder, then zip them together.You don't need a server.cs or a desription.txt file for faces and decals. I don't know why.
after looking at the zip, I think the image has to be the same. oh and make it 256x256. might help.