change or add thumbnail picture of theme

I am not sure if this is the right place to ask but, I want to know how to go about making and adding/changing the theme thumbnail picture that is shown appearance/themes section. every time I add a new child theme to the theme I am using I don’t get a picture of the themes screen shot, i,e. I am using the twentyeleven theme I would like the the child theme that I have activated to use the same twentyeleven screenshot. Any ideas?