If you are drawing on the Graphics of the Control than you should do something draw on the Bitmap everything you are drawing on the canvas, but have in mind that Bitmap needs to be the exact size of the control you are drawing on:
Bitmap bmp = new Bitmap(myControl.ClientRectangle.Width,myControl.ClientRectangle.Height);
Graphics gBmp = Graphics.FromImage(bmp);
gBmp.DrawEverything(); //this is your code for drawing
gBmp.Dispose();
bmp.Save("image.png", ImageFormat.Png);
Or you can use a DrawToBitmap method of the Control. Something like this:
Bitmap bmp = new Bitmap(myControl.ClientRectangle.Width, myControl.ClientRectangle.Height);
myControl.DrawToBitmap(bmp,new Rectangle(0,0,bmp.Width,bmp.Height));
bmp.Save("image.png", ImageFormat.Png);