Sometimes it is necessary to force xbmc / Kodi to update it’s saved images for TV Shows or Movies. Whilst you can do this individually by clicking on each item within the library, it can be a long process.
If you navigate to the userdata/Database folder and delete the texture*.db files and also delete the userdata/Thumbnails folder.
XBMC will now recreate the files / images on next startup.
If, the images are still wrong or not scraping correctly then you will need to remove the sources / directories from the library within the videos section, then clean the library, then re-add the sources / directories. On the next library update all of the information will be re-downloaded. All of the watched status etc should still be stored safely.
