You could have all your picture boxes in a list, as well as a class-level integer variable holding current picturebox index. On click of the button, just increment or decrement the index, and set focus on corresponding picturebox.
Dim list As New List(Of PictureBox)
Dim currentIndex As Integer = 0
Private Sub NextButton_Click(sender As Object, e As EventArgs) Handles NextButton.Click
currentIndex = (currentIndex + 1) Mod list.Count
list(currentIndex).Focus()
End Sub
Private Sub PreviousButton_Click(sender As Object, e As EventArgs) Handles PreviousButton.Click
currentIndex = (currentIndex - 1) Mod list.Count
list(currentIndex).Focus()
End Sub