Hello,
Probably you did not check the style.css file of the parent theme attentively because these images are called from style.css, for example http://prntscr.com/oljfka
So, if you want to overwrite the existing bg add the custom code to child theme style.css, for example
.side-area .close-block {
background-image: url(link_to_new_image_in_child_theme);
background-position: 20px 50px;
}
Regards