In your Hero model, you have an image field.:

headshot = models.ImageField(null=True, blank=True, upload_to="hero_headshots/")
By default it shows up like this:
_images/imagefield.png
You have been asked to change it to that the actual image also shows up on the change page. You can do it likethis:
@admin.register(Hero)
class HeroAdmin(admin.ModelAdmin, ExportCsvMixin):

    readonly_fields = [..., "headshot_image"]

    def headshot_image(self, obj):
        return mark_safe('<img src="{url}" width="{width}" height={height} />'.format(
            url = obj.headshot.url,
            width=obj.headshot.width,
            height=obj.headshot.height,
            )
    )
With this change, your imagefield looks like this:
_images/imagefield_fixed.png