I’m using version 22.06.23, downloaded today.
I see a few similar questions over the years.
Open a color clip by going to Open Other or by using the shortcut keys Ctrl+Shift+O. Choose Color. In the window you’ll see it says “transparent”. Click the button to the left of it that says Color. Choose white then press the Okay button. Shotcut will open a color clip in the video player under the Source tab.
Bring that color clip to your timeline and place it on v1 and extend it for as long as you want your video to last. Place all of your pictures on video tracks above v1. Now, you will have a white background for your video.
The problem is this:
v3 is on top, unless v1 is visible, then gets covered by intersection portions of v2
v2 is underneath v3 unless v1 is visible
v1 contains WHITE color
In other words,
v3 behave as expected, unless
v1 has a color, in which case
v2 is obscuring
I saw mention of “composite” setting in a post from 2017, but it doesn’t look still relevant.