private void Form1_Load(object sender, System.EventArgs e)
Bitmap newBmp = new Bitmap(100, 100);
Graphics g = Graphics.FromImage(newBmp);
g.FillRectangle(new SolidBrush(Color.Red), 0, 0, 33, 100);
g.FillRectangle(new SolidBrush(Color.White), 34, 0, 33, 100);
g.FillRectangle(new SolidBrush(Color.Blue), 68, 0, 33, 100);
pictureBox1.Image = newBmp; //pictureBox1 was dropped on the form
Change the control's Anchor property so that it is anchored on all 4 sides.
Please note that you can only have 1 control per form anchored in this manner (all 4 sides). And other controls on the form should be anchored by their sides that are not adjacent to special control anchored on all 4 sides.
G. G. Arun Ganesh discusses these properties in Working with Anchoring and Docking Properties in C# on C# Corner.