images = [Image.open(filename).resize((300,300)) for filename in existing_images_df['impath']]

image_width, image_height = images[0].size

one_square_size = int(np.ceil(np.sqrt(len(images))))
master_width = (image_width * one_square_size)
master_height = image_height * one_square_size

master = Image.new(
    mode='RGBA',
    size=(master_width, master_height),
    color=(0,0,0,0))  # fully transparent

for count, image in enumerate(images):
    div, mod = divmod(count,one_square_size)
    h_loc = image_width*div
    w_loc = image_width*mod
    master.paste(image,(w_loc,h_loc))

master.convert("RGB").save('sprite.jpg', transparency=0)