In addition to the excellent advice Sergey gave you, here, I'd like to add that once you do get your ToolTip created (once), and it's reusable: then you can write your MouseHover EventHandler like this:
private void pictureBox_MouseHover(object sender, EventArgs e)
{
Control theControl = sender as Control;
if(theControl != null)
{
ToolTips(theControl);
}
}
While you can correctly assume the test for 'null is not really needed, implementing it won't hurt anything and may just be helpful in debugging if there is a problem.
I assume that you are assigning every Control that you link to this MouseHover EventHandler a Tag property ... correct ?