Fastest way to clear the cache is:
systemctl stop mediacenter
rm -fr /home/osmc/.kodi/userdata/Thumbnails/*
rm /home/osmc/.kodi/userdata/Database/Textures13.db
systemctl start mediacenter
Next, enable the Kodi webserver (Settings->Services->Remote Control). Pick a username and password.
Then, install the texture cache maintenance utility. The default settings will likely work, although you will have to set a username and password from what you set in Kodi.
Then, run the following:
texturecache.py c movies
texturecache.py c sets
texturecache.py c tvshows