[grim]

I'm not exactly sure what you are asking, but I think it is this:

You want to place a decal and then a transparent plate over it?

If that is the case then apply a decal as normal (create a sketch, insert an image, execute the decal command) then create a part that has it's material set to glass and place it over it.

If you want to place the transparent plate over the decal within a part then you will need to change the properties of each face to glass.