Sorry I am very slow but.. (in Windows)

I suppose that is line command (that is a nightmare for me :)

so

bild.png is your image of the first post
pass.txt is a file with only "Hiergibtesnichtszusehen." (without "" inside)
both files are on the desktop

I put this in my Line command (Key Logo Windows + R) in windows 10
C:\Users\Pilou\Desktop\convert bild.png -decipher pass.txt image.png
and launch...
I have a message "convert is not found" !

When
C:\Users\Pilou\Desktop\bild.png -decipher pass.txt image.png
I have only your image of the first post at the screen and not image.png created
(who must i suppose different than the first image ? :)

There is something that i missed! :)
I have not installed anything more!

Suspense is intolerable! :)

PS And I am not sure that you have right "legally" to use such thing following country where you are ?