Solved by ptrblck in post #3 
              
              
                It looks like some of your images are RGB (3 channels) images while others might be in grayscale format (1 channel). 
Could you try to convert all the RGB using: 
item_A = self.transform(Image.open(self.files_A[index % len(self.files_A)]).convert('RGB'))
item_B = self.transform(Image.open(self.files…