Pieterbroeckx on "[Theme: Twenty Eleven] make img footer responsive"

do not change original themplate
try this in the child theme style.css

/*hide meta bottom*/
.singular footer.entry-meta {
	display: none;
}
/*image in footer*/
#site-generator {
	-webkit-background-size: cover;
	-moz-background-size: cover;
	-o-background-size: cover;
	background-image: url("/wp-content/uploads/dcf_footer.jpg");
	padding: 50px;
	background-repeat: no-repeat;
	background-position: center center;
	background-size: 100%;
	/*hide wordpress add*/
	text-indent: -9000px;
}
/*remove space between content and footer*/
#supplementary {
	padding: 0em 0%;
}