'Contiene le PictureBox e i loro limiti rispetto alla PictureBox grande
Dim pictRect As Dictionary(Of PictureBox, Rectangle)

Protected Overrides Sub OnLoad(ByVal e As System.EventArgs)
 MyBase.OnLoad(e)
 pictRect = New Dictionary(Of PictureBox, Rectangle)(100)
 For Each pictBox As PictureBox In Me.Controls
  'non bisogna aggiungere la PictureBox grande
  If Not pictBox Is Me.pictureBoxGrande Then
   'Poichè la posizione è data rispetto al controllo padre...
   Dim location As Point = pictBox.Location
   '... Eseguo un offset per ottenerla rispetto alla PictureBox grande
   location.Offset(-pictureBoxGrande.Location.X, -pictureBoxGrande.Location.Y)
   'Infine aggiungo PictureBox e Rettangolo al Dictionary
   pictRect.Add(pictBox, New Rectangle(location, pictBox.Size))
  End If
 Next
End Sub

Private Sub pictureBoxGrande_MouseMove(ByVal sender As System.Object, ByVal e As System.Windows.Forms.MouseEventArgs) Handles pictureBoxGrande.MouseMove
 'Ciclo tutti i rettangoli nel Dictionary
 For Each rect As Rectangle In pictRect.Values
  If rect.Contains(e.Location) Then
   Cursor = Cursors.Hand
   Return
  End If
 Next
 Cursor = Cursors.Default
End Sub

Private Sub pictureBoxGrande_MouseClick(ByVal sender As System.Object, ByVal e As System.Windows.Forms.MouseEventArgs) Handles pictureBoxGrande.MouseClick
 'Ciclo tutte le PictureBox nel Dictionary
 For Each pictBox As PictureBox In pictRect.Keys
  'Controllo se il rettangolo relativo alla PB grande contiene il cursore
  If pictRect(pictBox).Contains(e.Location) Then
   'Se si mostro la PictureBox
   pictBox.BringToFront()
   Return
  End If
 Next
End Sub