WordPress display image link in shortcode

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>";
}