Stamina,
Make some copies of deafult decal and overlay images and open them in a paint program and see how they are constructed. To have transaprency you need an alpha channel. To have an animated decal look at something like the smoke decal and you will see how the animation is created with a series of images.
Copy and study too the associated script files.
From an understanding of those you can make your own with original names and set them up following the default guidelines.
A more detailed explanation of how to make and execute them effectively will take a much longer post which I am not getting into here.
You will need to learn for yourself anyway as making new ones can be trickey and individual ones may require some dexterity from the user different possibly in each case to get them to work exactly as you wish.
