Compare the page image of https://en.wikipedia.org/wiki/Fantasy_Pieces_for_Clarinet_and_Piano_(Schumann):
On the latter page the thumbtime parameter is used to set the frame of the video to a more relevant thumbnail (a still of Narek Hakhnazaryan) but the page image remains the default thumbnail.
There are 3 places in PageImages which render the page image (API, action=info and getPageImage method)
We can pass the thumbtime from parameters to a stored page prop for every video.
Once this is in place we can modify the 3 places that render
https://gerrit.wikimedia.org/r/#/c/mediawiki/extensions/PageImages/+/442741 demonstrates and confirms this is feasible for the action=info render.