maybe if we look at the problem from another angle, we could solve it otherwise, out of the box. signals are very handy when it comes to handle images (add, update and delete) and below how i managed to solve your issue:

in models.py:

# from django.template.defaultfilters import slugify


class Post(models.Model):
    id = models.UUIDField(primary_key=True, default=uuid.uuid4, editable=False)
    author = models.ForeignKey(User, on_delete=models.CASCADE)
    title = models.CharField()
    # slug = models.SlugField('slug', max_length=255,
        # unique=True, null=True, blank=True,
        # help_text='If blank, the slug will be generated automatically from the given title.'
    # )
    content = models.TextField(blank=False)

    # ------------------------------------------------------------------------------------

    # rename images with the current post id/pk (which is UUID) and keep the extension
    # for cover thumbnail we append "_thumbnail" to the name 

    # e.g: 
    # img/posts/77b122a3d241461b80c51adc41d719fb.jpg
    # img/posts/77b122a3d241461b80c51adc41d719fb_thumbnail.jpg

    def upload_cover(instance, filename):
        ext = filename.split('.')[-1]
        filename = '{}.{}'.format(instance.id, ext)
        path = 'img/posts/'
        return '{}{}'.format(path, filename)

    postcover = models.ImageField('Post Cover',
        upload_to=upload_cover,  # callback function
        null=True, blank=True,
        help_text=_('Upload Post Cover.')
    )

    def upload_thumbnail(instance, filename):
        ext = filename.split('.')[-1]
        filename = '{}_thumbnail.{}'.format(instance.id, ext)
        path = 'img/posts/'
        return '{}{}'.format(path, filename)

    postcover_tn = models.ImageField('Post Cover Thumbnail',
        upload_to=upload_thumbnail,  # callback function
        null=True, blank=True,
        help_text=_('Upload Post Cover Thumbnail.')
    )

    # ------------------------------------------------------------------------------------

    published_date = models.DateTimeField(auto_now_add=True, null=True)

    def save(self, *args, **kwargs):

        # i moved the logic to signals

        # if not self.slug:
            # self.slug = slugify(self.title)
        super(Post, self).save(*args, **kwargs)

create new file and rename it signals.py (near to models.py):

import io
import sys

from PIL import Image

from django.core.files.uploadedfile import InMemoryUploadedFile
from django.dispatch import receiver
from django.db.models.signals import pre_save, pre_delete

from .models import Post

#  DRY
def image_resized(image, h):
    name = image.name
    _image = Image.open(image)
    content_type = Image.MIME[_image.format]
    r = h / _image.size[1]  # ratio
    w = int(_image.size[0] * r)
    imageTemproaryResized = _image.resize((w, h))
    file = io.BytesIO()
    imageTemproaryResized.save(file, _image.format)
    file.seek(0)
    size = sys.getsizeof(file)
    return file, name, content_type, size


@receiver(pre_save, sender=Post, dispatch_uid='post.save_image')
def save_image(sender, instance, **kwargs):

    # add image (cover | thumbnail)
    if instance._state.adding:

        #  postcover
        file, name, content_type, size = image_resized(instance.postcover, 500)
        instance.postcover = InMemoryUploadedFile(file, 'ImageField', name, content_type, size, None)

        #  postcover_tn
        file, name, content_type, size = image_resized(instance.postcover_tn, 175)
        instance.postcover_tn = InMemoryUploadedFile(file, 'ImageField', name, content_type, size, None)


    # update image (cover | thumbnail)
    if not instance._state.adding:
        # we have 2 cases:
        # - replace old with new
        # - delete old (when 'clear' checkbox is checked)

        #  postcover
        old = sender.objects.get(pk=instance.pk).postcover
        new = instance.postcover
        if (old and not new) or (old and new and old.url != new.url):
            old.delete(save=False)

        #  postcover_tn
        old = sender.objects.get(pk=instance.pk).postcover_tn
        new = instance.postcover_tn
        if (old and not new) or (old and new and old.url != new.url):
            old.delete(save=False)


@receiver(pre_delete, sender=Post, dispatch_uid='post.delete_image')
def delete_image(sender, instance, **kwargs):
    s = sender.objects.get(pk=instance.pk)

    if (not s.postcover or s.postcover is not None) and (not s.postcover_tn or s.postcover_tn is not None):
        s.postcover.delete(False)
        s.postcover_tn.delete(False)

in apps.py:

we need to register signals in apps.py since we use decorators @receiver:

from django.apps import AppConfig
from django.utils.translation import ugettext_lazy as _


class BlogConfig(AppConfig):  # change to the name of your app
    name = 'blog'  # and here
    verbose_name = _('Blog Entries')

    def ready(self):
        from . import signals

and this is the first screen shot of post admin area

since the thumbnail is generated from post cover, as UI/UX good practices there's no need to show a second input file for post cover thumbnail (i kept the second image field read only in admin.py).

below is the second screenshot after i uploaded the image

PS: the screenshot is taken from another app that i'm working on, so there's little changes, in your case you should see

• Post Cover instead Featured Image
• Currently: img/posts/8b0be417db564c53ad06cb493029e2ca.jpg (see upload_cover() in models.py) instead Currently: img/blog/posts/featured/8b0be417db564c53ad06cb493029e2ca.jpg

in admin.py

# "img/posts/default.jpg" and "img/posts/default_thumbnail.jpg" are placeholders
# grab to 2 image placeholders from internet and put them under "/static" folder

def get_post_cover(obj):
    src = obj.postcover.url if obj.postcover and \
    hasattr(obj.postcover, 'url') else os.path.join(
        settings.STATIC_URL, 'img/posts/default.jpg')
    return mark_safe('<img src="{}" height="500" style="border:1px solid #ccc">'.format(src))
get_post_cover.short_description = ''
get_post_cover.allow_tags = True

def get_post_cover_thumbnail(obj):
    src = obj.postcover_tn.url if obj.postcover_tn and \
    hasattr(obj.postcover_tn, 'url') else os.path.join(
        settings.STATIC_URL, 'img/posts/default_thumbnail.jpg')
    return mark_safe('<img src="{}" height="175" style="border:1px solid #ccc">'.format(src))
get_post_cover_thumbnail.short_description = ''
get_post_cover_thumbnail.allow_tags = True


class PostAdmin(admin.ModelAdmin):
    list_display = ('title', .. )
    fields = (
        'author', 'title', 'content',
        get_post_cover, get_post_cover_thumbnail, 'postcover',
    )
    readonly_fields = (get_post_cover, get_post_cover_thumbnail)

[..]

and finally you don't need any delete logic in save() function in forms.py