Switching between two forms

I will have form1 which will always remain open. Then I have my form2 which gets displayed when user clicks a button in form1 and populates some picture in form2. Later when the user finishes viewing form2 and selects form1 again from taskbar, form2 must be closed automatically. This routine has to be repeated for a particualr button click event from form1.

Which event do I have to add the code to show and hide my form2?

Form1.Activated or Form1.Click or Form1.Enter or and so on.. Which would be the best event?