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How am I supposed to use it? I tried it and they're only showing me different images of my same picture, how does it works?
The Postmark Reveal Website is doing its job. That is, it takes your image and shows you different manipulations in an attempt to better show you the Postmark. It usually works, but sometimes not. The problem is that you don't have the stamp, so we only have an image behind plastic, which is not helping Postmark Reveal do its job. When I tried your image, the best I got was the following, but you can see the Maltese Cross is blurry:

Just as an aside, here is my Penny Black with a similar Maltese Cross, scanned directly (not in plastic):

Postmark Reveal can do a better job with a clean scan:
