Remove it from the Controls array and reinsert it at the end:
Controls.Remove(myPictureBox)
Controls.Add(myPictureBox)
The last added object will be on top of the others.
Alternatively, sort them into the order you do want, and add them all at once in that order.
Remember that you can use the SQL ORDER BY clause to select the order in which records are returned from your DB as well.