from django.contrib import messages
class YourModeAdmin(admin.ModelAdmin):
...
def save_model(self, request, obj, form, change):
#不保存大图片
if obj.picture and obj.picture.size > 20480:
messages.set_level(request, messages.ERROR)
messages.error(request, 'The picture\'s too large. It\'s supposed smaller than 20K.')
else:
obj.save()