Some skins in XBMC allow extra fanart. These images are put into an "extrathumbs" subfolder under the related movie. As such extrathumbs options will only be visible with certain folder structure preference settings.
Extrathumbs have been added to the movie fanart tab. In addition another preference has been added to the movie preferences. There are 3 folder formats that MC now understands selectable by the preference.
1. Movies can be in the root & in subfolders (extrathumbs not available)
2. Movies are in folders but you still use the movie name for identification (extrathumbs are possible)
3. Movies are in folders but you use the foldername to determine the movie name (extrathumbs are possible)
At the moment MC can save & display 4 extrathumbs in addition to the main fanart. You can also use the full options for fanart - Hires/Lores/Url/Browse & Crop.
MC will create the extrathumbs folder if it does not already exist.
CheersMC Dev Team