If something is right, show me one picture. If something ist false show me another picture. JS, HTML5, CSS

I'm new in HTML5, CSS3 and JS.. now i wanted to create a webpage with a formular where someone can put some text in it. If the entered text is right, one picture (gif) should be shown. If the text is false, another picture should be shown. Now is my question: How can i do this? And how can i style the formular in CSS?