Let's go over the general idea here. I'm not going to even bother with how you're doing things right now, cause it's not going to do what you want. Period.
All you need is two PictureBoxes, one for each die. You then have 6 images, one for each possible value. I'd suggest keeping the images in an array or perhaps an ImageList (either way, let's call it images); it will make things much simpler.
When you roll, for each die, you'll say something like roll = rand.Next(6);. roll will then correspond to the index of the image in the array. You set the Image of the corresponding PictureBox to images[roll] (or images.Images[roll] if you're using an ImageList). No need to mess with Visible; the two PictureBoxes will always be visible.
Just be aware that when you consider the actual value of roll, it will be from 0 to 5. Add 1 to get the value people expect to see.
