You can reuse the $content
parameter:
function image_code($atts, $content = null) {
$url = esc_url( $content );
return "<a href="https://wordpress.stackexchange.com/questions/78252/$url"><img src="https://wordpress.stackexchange.com/questions/78252/$url" class="user-imgs" /></a>";
}
Or pass the URL as parameter in case you want to use a different URL:
function image_code($atts, $content = null) {
$args = shortcode_atts( array( 'url' => FALSE ), $atts );
$img = esc_url( $content );
$url = $args['url'] ? esc_url( $args['url'] ) : $img;
return "<a href="https://wordpress.stackexchange.com/questions/78252/$url"><img src="$img" class="user-imgs" /></a>";
}