Items and blocks can't be put in an item frame with the off-hand

    public boolean processInitialInteract(EntityPlayer player, EnumHand hand)
    {
        ItemStack itemstack = player.getHeldItem(hand);

//      if (!this.world.isRemote)
//      {
            if (this.getDisplayedItem().isEmpty())
            {
                if (!itemstack.isEmpty())
                {
                    this.setDisplayedItem(itemstack);

                    if (!player.capabilities.isCreativeMode)
                    {
                        itemstack.shrink(1);
                    }

++                  return true;
                }
				
++		return false;
            }
            else
            {
                this.playSound(SoundEvents.ENTITY_ITEMFRAME_ROTATE_ITEM, 1.0F, 1.0F);
                this.setItemRotation(this.getRotation() + 1);
            }
//      }

        return true;
    }