I haven't found a solution to this for python...if you have access to the computer running Tableau Server, it looks like you are able to clear the cache using a tabadmin or redis command (see link for more). tabadmin requires server outages...not sure if the redis route would.
I'm still testing out the image endpoint, so I've worked around image caching by duplicating the views on tableau server and pinging the server for the duplicate views...not sure if your application is more mature.
I read that the vizportal cache is not the same one as the server cache. Not sure if that's true.
Since I need the cached image soon after updating the source. Would changing vizportal.rest_api.view_image.max_age to 5 minutes be loading the image too frequently? I'm worried about slowing down the server