As long as you keep in mind that the picture and poster images together form a background
for replacing the green of the green screen.
As koala explained, if you want to move your arm in front of any background images or video, it must be clear what is YOU and what is BACKGROUND.
You have never seen this image before but instantaneously you see what is the woman and what is the background, thanks to your brain analyzing the picture. But a computer lacks that superpower. It only has 144688 bytes and has no clue which of them belong to Woman and which belong to Background. So if you want to separate Woman from the rest, the computer has to be helped.
You can try to stand perfectly still and then tell the computer exactly which pixels belong to Woman and which don't... But you don't want to do that, rather you want to move your arm. So to make things a bit easier telling which pixels belong to Woman or not: use a green screen.
Now you just say: this specific kind of green is background, I want that to be made transparent. All the rest is Woman.
The next step is filling the now transparent part with anything else. If you just returned from a trip abroad and want to show, take a landscape with balloons, it can be a fullscreen movie. If you are in a more tutorial-ish setting and would rather use your room, use a picture of that (without you in the picture). It might be convenient to just right click OBS Studio's Preview or Program window, the one currently having your webcam, and click
Screenshot (image will be saved in your default video output folder, see
Settings).
Long story short, if you want to move around in front of any other background than the real one being captured by your cam, you'd have to use a green screen or similar solution to split. And of course, if that's too much of a hassle, you can also avoid it. Place yourself off centre and just point at the images if applicable.
