Every control has a Controls collection. Just as you get the controls on a form from the form's Controls collection, you get the controls on a Panel from the Panel's Controls collection.
Exactly how I just told you: from the Controls collection. If the Panel contains other controls too then it might be easier to maintain a separate List(Of PictureBox). That way you can just loop through that and you'll know that you'll get the PictureBoxes, all the PictureBoxes and nothing but the PictureBoxes.