Yes the MouseHover event does not use the MouseEventHandler so you don't get the e.x and e.y. Another way to do this is to get the Cursors position, and since your in a form use PointToClient to get the correct location.
private void picturebox1_MouseHover(object sender, EventArgs e)
{
ToolTip tt = new ToolTip();
Point p = pictureBox1.PointToClient(System.Windows.Forms.Cursor.Position);
if (p.X >= 18 && p.X <= 71 && p.Y >= 21 && p.Y <= 79)
{
tt.SetToolTip(this.picturebox1, "something");
}
else
{
tt.Hide(this.picturebox1);
}
}