Given a tensorflow event file, how can I extract images corresponding to a specific tag, and then save them to disk in a common format e.g. .png?

You could extract the images like so. The output format may depend on how the image is encoded in the summary, so the resulting write to disk may need to use another format besides .png

import os
import scipy.misc
import tensorflow as tf

def save_images_from_event(fn, tag, output_dir='./'):
    assert(os.path.isdir(output_dir))

    image_str = tf.placeholder(tf.string)
    im_tf = tf.image.decode_image(image_str)

    sess = tf.InteractiveSession()
    with sess.as_default():
        count = 0
        for e in tf.train.summary_iterator(fn):
            for v in e.summary.value:
                if v.tag == tag:
                    im = im_tf.eval({image_str: v.image.encoded_image_string})
                    output_fn = os.path.realpath('{}/image_{:05d}.png'.format(output_dir, count))
                    print("Saving '{}'".format(output_fn))
                    scipy.misc.imsave(output_fn, im)
                    count += 1  

And then an example invocation may look like:

save_images_from_event('path/to/event/file', 'tag0')

Note that this assumes the event file is fully written -- in the case that it's not, some error handling is probably necessary.