@eyllanesc png works in both versions. with svg, using
set QT_DEBUG_PLUGINS=1
python3.9 my_file.py
when checking the box it fails with "Cannot load library [...]\\Roaming\\Python\\Python39\\site-packages\\PySide6\\plugins\\imageformats\\qsvg.dll: Das angegebene Modul wurde nicht gefunden. [cannot find the module]" besides other similar errors (even though it exists). When I run it with Python3.8 it works.
However, I noticed that PySide2/6 (and weirdly I think also the Python 3.9 installation) was located in Roaming for Python 3.9 and that it seems to look in various places for packages and the paths for site-packages in Roaming differ from the paths in AppData/Local/Programs. Therefore, although there was only one global (PATH) Python 3.9 installed, I tried by removing the Roaming stuff and reinstalled Python 3.9 and the packages, and now it seems to work...