How to take Whatsapp Screenshot in Samsung S duos 6802?

 I have Samsung S Duos 6802. Except some lag, it works fine most of the times. I was chatting on whatsapp with my girlfriend when she messaged me the screenshot of our chat. It was my photo in the background of the chat. I was very happy with such a romantic gesture. Suddenly she asked for screenshot of my mobile. She asked if I could do such a thing for her. I agreed and set a very nice pic of her as the background. But then I realized, I DON'T KNOW HOW TO TAKE THE SCREENSHOT. I searched for it on Google. But none of the searches helped me. Most of the searches were about other models. I want to send her the screenshot to make her smile. Please help me.