Can the video be just voice and my PC screen?
yes sure...
I guess most of udemy courses are screen cast and voice over
If it is suitable for you it is better to have yourself on the video. I heard some stories that videos were pirated and reuploaded to the different sites. If you are in the picture it is easier to protect your work.
Hi, you can report those links where the videos have been pirated to Udemy.
Here's the Udemy's trust and safety link:
https://support.udemy.com/hc/en-us/articles/229604108-What-should-I-do-if-I-find-a-course-on-a-pirate-site-
I do almost all of my courses as voice over. So I think it's fine.
