Prevent django admin from escaping html

I'm trying to display image thumbnails in django admin's list_display and I am doing it like this:

from django.utils.safestring import mark_safe

class PhotoAdmin(admin.ModelAdmin):
    fields = ('title', 'image',)
    list_display = ('title', '_get_thumbnail',)

    def _get_thumbnail(self, obj):
        return mark_safe(u'<img src="%s" />' % obj.admin_thumbnail.url)

Admin keeps displaying the thumbnail as escaped html, although I marked the string as safe. What am I doing wrong?