I'm attempting to augment data using TF, but I've gone about things completely wrong. Can someone help me refactor my code?

So here's what I have written so far. You can probably see very quickly where things are going wrong. Please don't judge me too harshly.

# Adjust image brightness/contrast and add noise
pngs = ['foo.png', 'bar.png', ]
contrast_range = (0.9, 1.1)
brightness_range = (-0.025, 0.025)
noise_range = (0, 5)
augmentations_per_image = 5

with tf.Session() as sess:
    for png in tqdm(pngs):
        img = tf.image.decode_png(tf.read_file(png), channels=1)
        for aug_num in range(augmentations_per_image):
            # Get the particular augmentation values for this image
            delta_contrast = random.uniform(*contrast_range)
            delta_brightness = random.uniform(*brightness_range)
            delta_noise = random.uniform(*noise_range)

            # Generate the noise
            noise = tf.random_normal(
             shape=tf.shape(img), mean=0.0, stddev=delta_noise
            )
            noise = tf.cast(noise, tf.int32)

            # Apply the augmentations
            new_img = tf.image.adjust_contrast(img, delta_contrast)
            new_img = tf.image.adjust_brightness(new_img, delta_brightness)
            new_img = tf.cast(new_img, tf.int32)
            new_img = tf.add(new_img, noise)
            new_img = tf.clip_by_value(new_img, 0, 255)
            new_img = tf.cast(new_img, tf.uint8)

            # Save image
            new_img = tf.image.encode_png(new_img)
            filename = png.replace('.png', '_aug_{}.png'.format(aug_num))
            write_new_png = tf.write_file(filename, new_img)
            sess.run(write_new_png)

As you can probably tell, because I keep adding new graph nodes in the session, the program quickly grinds to a halt. From what I understand, I need to move the graph-defining parts outside of the session, and then pass each image through the pipeline, so to speak.

However, I'm not really sure how to do that. I tried spinning off the tf.___ bits into their own functions, but I ran into problems with those functions not being called, or sess.run complaining about return values. I then tried moving all the new_img bits outside of the session into the global scope, but then I couldn't figure out how to pass my img variable into that, so I kept getting NameErrors.

Basically, I'm just pretty clueless. I haven't really done much with TF, but my coworker left, and there's no one else to do it, so here I am. Can someone point me in the right direction for what I'm trying to do? I'm not asking for you to fix my code, I'd just like to get a sense of how I'm supposed to tackle this problem, and then I can handle the details myself.

Thanks, and apologies for the dumb question.