The VST thumbnail screenshots for Reason 9.5 and later are located here:
~/Library/Application Support/Propellerhead Software/Reason/Plugin Screenshots (macOS)
%LOCALAPPDATA%\Propellerhead Software\Reason (Windows)
Reason 11 Suite stores the screenshots in this folder instead:
~/Library/Application Support/Propellerhead Software/Reason Suite/Plugin Screenshots (macOS)
%LOCALAPPDATA%\Propellerhead Software\Reason Suite (Windows)
The easiest way to get your VST thumbnail screenshot over to the new destination would be to just copy the original folder content and move to the corresponding "Reason Suite" folder.