if you look in the html code you might recognize that the image you want to switch out doesn't have a class named 'meme'. You could just put 'img' instead of 'meme', but this would change the picture in the header too. So if you want to adress only this particular image you have to write '.main img'. This selects only the image in the div with the class 'main'.
Here the code:
var img = $(this).val();
$('.main img').attr('src', img);