Changing the database schema is always a bit problematic as this will cause sync-incompatibilities with app instances that aren't on the latest app version, that's why I try to keep those database schema changes to a minimum. Also, adding a thumbnail to the media table would add the question of that thumbnail should then also be synced to other devices - which then could make the sync more complex.
But a more simple solution just came to my mind: The thumbnail could just be cached locally, in the temp
-folder of the app. When first requesting the thumbnail, it would be generated and cached, all subsequent requests will just use the cache. Will try this out and see if it works well