Adding on to @dominic, I wanted to use this in multiple models, so I created a function that I could call in each model, inputing the image to be displayed.

For instance in one app I have:

from django.contrib import admin

from .models import Frontpage
from ..admin import image_file


class FrontpageAdmin(admin.ModelAdmin):
    list_display = ('image_thumb', ...)

    image_thumb = image_file('obj.image()') 

admin.site.register(Frontpage, FrontpageAdmin)

with image a function of Frontpage that returns an image.

In another app I have:

from django.contrib import admin

from .models import Exhibition
from ..admin import image_file


class ExhibitionAdmin(admin.ModelAdmin):
    list_display = ('first_image_thumb', ...)

    first_image_thumb = image_file('obj.related_object.image',
                                   short_description='First Image')

admin.site.register(Exhibition, ExhibitionAdmin)

This allows me to specify the image object and the short_description while keeping the boilerplate in another file. The function is:

from sorl.thumbnail import get_thumbnail
from django.conf import settings


def image_file(image, short_description='Image'):
    def image_thumb(self, obj):
        image = eval(image_thumb.image)
        if image:
            thumb = get_thumbnail(image.file, settings.ADMIN_THUMBS_SIZE)
            return u'<img width="{}" height={} src="{}" />'.format(thumb.width, thumb.height, thumb.url)
        else:
            return "No Image"

    image_thumb.__dict__.update({'short_description': short_description,
                                 'allow_tags': True,
                                 'image': image})
    return image_thumb

Add a method in your model (models.py):

def image_tag(self):
    return u'<img src="%s" />' % <URL to the image>
image_tag.short_description = 'Image'
image_tag.allow_tags = True

and in your ModelAdmin (admin.py) add:

readonly_fields = ('image_tag',)

Update v. 1.9

Note that in Django v.1.9

image_tag.allow_tags = True

is depricated and you should use format_html(), format_html_join(), or mark_safe() instead

So your model.py should look like this:

...
from django.utils.html import mark_safe

def image_img(self):
    if self.image:
        return mark_safe('<img src="%s" />' % self.image.url_125x125)
    else:
        return '(Sin imagen)'
    image_img.short_description = 'Thumb'

and in your admin.py add:

list_display= ('image_img','product',)
readonly_fields = ('image_img',)

and for adding it in the ‘Edit mode’ of your admin panel in your admin.py add:

fields = ( 'image_img', )

With django-imagekit you can add any image like this:

from imagekit.admin import AdminThumbnail

@register(Fancy)
class FancyAdmin(ModelAdmin):
    list_display = ['name', 'image_display']
    image_display = AdminThumbnail(image_field='image')
    image_display.short_description = 'Image'

    # set this to also show the image in the change view
    readonly_fields = ['image_display']

Since the allow_tags has been deprecated, you should use format_html function. The example code look like this:

models.py file:

class TestAdminModel(models.Model):

    img = models.URLField()

admin.py file:

from django.utils.html import format_html

class TestAdmin(admin.ModelAdmin):
    list_display = ["id", "img", "thumbnail"]

    def thumbnail(self, obj):
        return format_html('<img src="{}" style="width: 130px; 
                           height: 100px"/>'.format(obj.img))

    thumbnail.short_description = 'thumbnail'

admin.site.register(models.TestAdminModel, TestAdmin)

The result looks like this:

You can see more details from django documentation.