Coloca assim:
document.getElementsByClassName('tarjas')[0];
x = window.screen.width;
if (x <= 767) {
document.getElementsByClassName('slides')[0].getElementsByTagName('img')[0].src='https://cdn.awsli.com.br/500/500692/arquivos/banner-mobile-morfismo.jpg';
document.getElementsByClassName('slides')[0].getElementsByTagName('img')[1].src='https://cdn.awsli.com.br/500/500692/arquivos/banner-forest.jpg';
}