from arcgis.gis import GIS
from arcgis.gis import Item
gis = GIS('Pro')
itemID = 'YOU NEED TO FILL THIS OUT!!!'
new_thumbnail = r"YOU NEED TO FILL THIS OUT!!!"
#You can leave large thumb blank and it will add the sample thumbnail as both reg thumb and large thumb
#As regular thumbnail supports larger than 200x133 now, there may not be a need for large thumbnail
large_thumb = ''
large_thumb= large_thumb or new_thumbnail
item = Item(gis, itemID)
print (f'old Largethumb: {item.largeThumbnail}')
print (f'old thumb: {item.thumbnail}')
item.largeThumbnail = new_thumbnail
new_det = dict(largeThumbnail=large_thumb)
res = item.update(item_properties=new_det,thumbnail=new_thumbnail)
print (res)
item = Item(gis, itemID)
print (f'new Largethumb: {item.largeThumbnail}')
print (f'new thumb: {item.thumbnail}')