Choosing random picturebox out of 20 of them c#

I'm having a hard time with choosing random PictureBox out of 20 of it...

I want to make code that will choose 2 random PictureBox and set the same image for both of them, and should repeat that function 5 times...

I've tried using an array but there I got an error that says that pictureBox1 cant be transformed into an array.

string[] array1 = new string[]
                {
                    pictureBox1,


                };