admin.py

class ProductAdmin(admin.ModelAdmin):
    list_display = ('name','thumb')

    def thumb(self, obj):
        return  render_to_string('thumb.html',{
                    'image': obj.image
                })

    thumb.allow_tags = True

thumb.html

{% load thumbnail %}
{% thumbnail image '100x100' crop="top" as im %}
    <img src="{{ im.url }}" alt="">
{% endthumbnail %}