@paralis, Hi Frank, welcome to the forum!
If you haven't done yet, please read How to get the best out of this forum.
For the Displays section of the forum, please always post a link to the display in question, and to the processor board in your case. It makes it easier for responders to know exactly what you use.
Your panel might be this: GDEW075T7.
I consulted the panel specs for the "reference circuit", to know the correct value for the RESE resistor: 0.47 Ohms. Check if you have selected the correct setting of the switch on your driver board.
If you power your driver board from USB, make sure it is strong enough, and the voltage is high enough. USB3 ports are better suited with e.g. 5.2V. I think the LDO on the board is not of the low drop version, and the series diode not a shottky, so the voltage to the panel could be too low. Maybe this explains the distortions on your picture.
You could also test with the "normal" examples from Waveshare, instead of the WiFi picture loader. Or my example GxEPD2_WS_ESP32_Driver.ino.
Jean-Marc