I have a question about adding a personalized stamp among the annotations in Okular (setting Okular ... / annotation tools)

I tried this following the manual instructions and also as wrote by lueck in his/her contribute https://forum.kde.org/viewtopic.php?f=251&t=126704&p=335998&hilit=+okular+stamp#p335998
But the result is quite ZERO.
I can adds the personalized stamp in the annotation map and Okular recognizes it. But when I use it, I get only a transparent quadrilateral, sometimes empty or with a question mark within.

Have you any idea to let it run correctly?

Thanks in advance for helping

Nino

system data
Okular 17.04.2
KDE-Plasma 5.8.7
KDE-Frameworks 5.32.0
Qt vers. 5.6.2
OS = OpenSUSE Leap 42.3