You have to make for your form a member first:
private PictureBox picturebox1;
In the form constructor you can set the properties of the picturebox1 object. After this in the button click event you can use your PictureBox object without problem.
By the way you can iterate through the controls with foreach. Maybe you can find your dynamically created control as the follows:
foreach (var i in this.Controls)
{
if (i.GetType() == typeof(PictureBox))
{
Image p = i as Image;
p.Image =
}
}
I hope I could help you!