How to bind to a single entity a few pictures?

Good day !
There is a model:

class ShopCat(models.Model):
 title = models.CharField(max_length=100)

 def __unicode__(self):
 return self.title


class ShopItem(models.Model):
 cat_id = models.ForeignKey(ShopCat)
 title = models.CharField(max_length=100)
 sostav = models.TextField(max_length=1000)
 descr = models.TextField(max_length=1000)
 creation_date = models.DateTimeField(auto_now_add=True, auto_now=False)
 updated = models.DateTimeField(auto_now_add=False, auto_now=False, blank=True, null=True)
 price = models.IntegerField()

 def __unicode__(self):
 return self.title


class ShopItemImg(models.Model):
 ShopItem_id = models.ForeignKey(ShopItem)
 img = models.ImageField(upload_to='shop/', null=True, blank=True)

And admin.py write:

class ShopItemImgInline(admin.ModelAdmin):
 model = ShopItemImg
 extra = 3

class ShopItemAdmin(admin.ModelAdmin):
 fieldsets = [
 (None, {'fields': ['title', 'cat_id', 'price']}),
 ("Texts", {'fields': ['descr', 'sostav']}),
]

admin.site.register(ShopItem, ShopItemAdmin)
admin.site.register(ShopCat)
admin.site.register(ShopItemImg, ShopItemImgInline)

Everything works fine, BUT
When you try to ShopItemImgInline admin class(admin.ModelAdmin) change the ModelAdmin for StackedInline or TabularInline - error : AttributeError: 'ShopItemImgInline' object has no attribute 'urls'

Tell me - where nakosyachil ?