Updating thumbnails only

Am I correct that thumbnails specified in my feed are cached on Roku's side, and the Roku devices load (and potentially cache) thumbnails?

For instance, I have a feed, and later on I would like to update the thumbnail for one of the videos, but everything else stays the same. Am I correct that I can't just upload a new thumbnail file with the same name to my server and expect it to be picked up? (This seems to be the behavior I am seeing.)

If this is the case, what is the best way to update just a thumbnail graphic? 

Thank you!