Adding a source attribute to the picture markup

<?php

/*
Plugin Name: WP Tevko Responsive Images
Plugin URI: http://timevko.com
Description: Fully responsive image solution using picturefill and the ID of your image.
Version: 1.0.1
Author: Tim Evko
Author URI: http://timevko.com
License: MIT
*/


// First we queue the polyfill
function tevkori_get_picturefill() {
    wp_enqueue_script( 'picturefill', plugins_url( '/js/picturefill.js', __FILE__ ) );
}
add_action( 'wp_enqueue_scripts', 'tevkori_get_picturefill' );


// Add support for our desired image sizes - if you add to these, you may have to adjust your shortcode function
// TODO: Add UI for adjusting?
function tevkori_add_image_sizes() {
    add_image_size( 'large-img', 1000, 702 );
    add_image_size( 'medium-img', 700, 372 );
    add_image_size( 'small-img', 300, 200 );
}
add_action( 'plugins_loaded', 'tevkori_add_image_sizes' );

// alt tags will now be automatically included
function tevkori_get_img_alt( $image ) {
    $img_alt = trim( strip_tags( get_post_meta( $image, '_wp_attachment_image_alt', true ) ) );
    return $img_alt;
}

function tevkori_get_picture_srcs( $image, $mappings ) {
    $arr = array();
    foreach ( $mappings as $size => $type ) {
        $image_src = wp_get_attachment_image_src( $image, $type );
        $arr[] = '<source srcset="'. $image_src[0] . '" media="(min-width: '. $size .'px)">';
    }
    return implode( array_reverse ( $arr ) );
}

function tevkori_responsive_shortcode( $atts ) {
    extract( shortcode_atts( array(
        'imageid'    => 1,
        // You can add more sizes for your shortcodes here
        'size1' => 0,
        'size2' => 600,
        'size3' => 1000,
    ), $atts ) );

    $mappings = array(
        $size1 => 'small-img',
        $size2 => 'medium-img',
        $size3 => 'large-img'
    );

   return
        '<picture>
            <!--[if IE 9]><video style="display: none;"><![endif]-->'
            . tevkori_get_picture_srcs( $imageid, $mappings ) .
            '<!--[if IE 9]></video><![endif]-->
            <img src="' . wp_get_attachment_image_src( $imageid[0] ) . '" srcset="' . wp_get_attachment_image_src( $imageid[0] ) . '" alt="' . tevkori_get_img_alt( $imageid ) . '">
            <noscript>' . wp_get_attachment_image( $imageid, $mappings[0] ) . ' </noscript>
        </picture>';
}
// TODO: It this the best name? responsive_img? picture?
add_shortcode( 'responsive', 'tevkori_responsive_shortcode' );

// Alter Media Uploader output to output shortcode instead
// TODO: Make optional?
// TODO: Make this know what sizes are chosen, rather than hardcoded
function tevkori_responsive_insert_image( $html, $id, $caption, $title, $align, $url ) {
    return "[responsive imageid='$id' size1='0' size2='600' size3='1000']";
}
add_filter( 'image_send_to_editor', 'tevkori_responsive_insert_image', 10, 9 );