Tag doesn#t change in C#

I am trying to do a program where I choose out of 4 pictureboxes with images, that picturebox who has the image from the pixaleted picturebox. I am using a random function and because I don't want the picture to repeat itself, I used a Tag to indicate if the Image where used or not. The problem is that the tag doesn't change. I verified it with a Breakingpoint. I don't know what I have done wrong. Please help me. Thank you.

idx = r.Next(0, 6);
        pictureBox1.Image=Pixelate((Bitmap) Images[idx], 40);
        idp = p.Next(0, 3);
        pictureBoxs[idp].Image = Images[idx];
        Images[idx].Tag = "Used";
        pictureBoxs[idp].Tag = "Used";
        for(int i=0; i<4; i++)
        { 
            if (pictureBoxs[i].Tag != "Used")
            {
                    ind = r.Next(0, 6);
                    if (Images[ind].Tag != "Used")
                    {
                        pictureBoxs[i].Image = Images[ind];
                        Images[ind].Tag = "Used";
                    }

            }

        }