CSharp Code:
private List<Panel> panels = new List<Panel>();
private Dictionary<Panel, List<PictureBox>> pictureBoxesByPanel = new Dictionary<Panel, List<PictureBox>>();
Now when you add a Panel to the form you add it to the List and to the Dictionary along with an empty List<PictureBox>. When you add a new PictureBox to a Panel you get the appropriate List<PictureBox> from the Dictionary by specifying the appropriate Panel, then add the new PictureBox. Now you can access every PictureBox by the Panel it's on via the Dictionary.