This tutorial only works for face and body decals.*note* This tutorial was made on a Windows XP operating system, steps may be different for you *note*
First make sure you got the decals, which should be;
256 x 256 for face and body decals
In this example I will be using my Mythbusters decals.

You also want to create a folder called "thumbs" in the same place where you decals are.

Inside this folder, you should have thumbnails of your face and body decals.
64x64 in size. Make sure they are also named exactly the same as it's corresponding decal.

Now, if you have winrar (if not I recommend you go get it right
here), highlight all of the files like so;

Right click and select "Add to archive"

A box like so should open;

In the Archive name box, (
Green) you want to name it either
Face_*name*
or
Decal_*name*
Replace *name* with the name of your choice.
Be sure to select the .zip box (
Red) or else the decals will not show up in-game.
So for my Mythbusters decals, this is how it will look:

After this, click Ok and a .zip file should appear in the same place where your original files are. Place this .zip file into your Blockland add-ons folder and you're all done.
Enjoy.