I just switched both keys ACUserPreferencePolicy and DCUserPreferencePolicy to 0 under Computer\HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Control\Class\\0000, rebooted my computer, and voila, no more weird automatic brightness changing! In order for there to be absolutely no brightness changing when images are switched, also disable HDR in the Windows display settings. Intel for some reason doesn't give you the option to switch off automatic brightness adjustment in the Graphics Command Center on AC power. T his fix actually worked when nothing else did.
You have to edit two simple registry entries on Windows. I solved it thanks to a post on SuperUser for good. I have a Lenovo S740-15IRH and I also experienced a weird auto-brightness adjustment when switching between dark and light images on screen that was very annoying.
