i'd like to render a piece of paper, with a layer of transparent, glased ink. (see images below)

The thing is, i can't tune down the "alpha" settings in the Materials pannel, as it is already used by the "Images as planes" thing. 
Every time i try to change something in the Materials pannel, it goes wild because of the transparency of the png image. I am using cycles. 

I'm a beginner in Blender and would love to see how you guys would do it. 
Thanks a lot!

[![enter image description here][1]][1]
[![enter image description here][2]][2]


  [1]: https://i.sstatic.net/6Ijjm.jpg
  [2]: https://i.sstatic.net/4715s.jpg

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Here is an image to clarify :)
[![enter image description here][1]][1]

Here is the file:
[<img src="https://blend-exchange.giantcowfilms.com/embedImage.png?bid=rZ1m8BvL" />](https://blend-exchange.giantcowfilms.com/b/rZ1m8BvL/)


  [1]: https://i.sstatic.net/6yb27.jpg