Reason 11 Suite stores the VST thumbnail screenshots in this folder:
~/Library/Application Support/Propellerhead Software/Reason Suite/Plugin Screenshots (macOS)
%LOCALAPPDATA%\Propellerhead Software\Reason Suite (Windows)
Reason 12 looks for VST thumbnail screenshots here:
~/Library/Application Support/Propellerhead Software/Reason/Plugin Screenshots (macOS)
%LOCALAPPDATA%\Propellerhead Software\Reason (Windows)
The easiest way to get your VST thumbnail screenshot over to the new destination would be to just copy the original "Reason Suite" folder content and paste in the corresponding "Reason" folder.