<div class="class1" style="background-image:url(https://www.awebsite.com/sites/default/files/a_image.jpg?t=1529620960)"></div>

https://www.awebsite.com/sites/default/files/a_image.jpg?t=1529620960`

https://hello.com/an-other-image.jpg

c = document.getElementsByClassName('class1');
for(i = 0; i < c.length; i++){
    c[i].style.backgroundImage = 'url(https://hello.com/an-other-image.jpg)';
}

// querySelector can use CSS selectors to find the element in the document
// note: this is assuming there is only 1 element on page with this class
var myDiv = document.querySelector('.class1');
// Style properties generally match CSS but camel cased
myDiv.style.backgroundImage = 'url("https://hello.com/an-other-image.jpg");