Put this in MouseMove of the PictureBox:

Code:
//Declarations of few variables to minimize the number of refers to the Picture Box.

{
	int imgWidth = PictureBox1.Image.Width;
	int imgHeight = PictureBox1.Image.Height;
	int boxWidth = PictureBox1.Size.Width;
	int boxHeight = PictureBox1.Size.Height;

	//This variable will hold the result
	float X = e.X;
	float Y = e.Y;
	//Comparing
...