<?php 

add_action( 'admin_init', 'add_post_gallery_so_14445904' );
add_action( 'admin_head-post.php', 'print_scripts_so_14445904' );
add_action( 'admin_head-post-new.php', 'print_scripts_so_14445904' );
add_action( 'save_post', 'update_post_gallery_so_14445904', 10, 2 );

/**
 * Add custom Meta Box to Posts post type
*/
function add_post_gallery_so_14445904()
{
	add_meta_box(
	'post_gallery',
	'Studio Image Uploader',
	'post_gallery_options_so_14445904',
	'page',// here you can set post type name
	'normal',
	'core'
			);
}

/**
 * Print the Meta Box content
 */
function post_gallery_options_so_14445904()
{
	global $post;
	$gallery_data = get_post_meta( $post->ID, 'gallery_data', true );

	// Use nonce for verification
	wp_nonce_field( plugin_basename( __FILE__ ), 'noncename_so_14445904' );
	?>

<div id="dynamic_form">

    <div id="field_wrap">
    <?php 
    if ( isset( $gallery_data['image_url'] ) ) 
    {
        for( $i = 0; $i < count( $gallery_data['image_url'] ); $i++ ) 
        {
        ?>

        <div class="field_row">

          <div class="field_left">
            <div class="form_field">
              <label>Image URL</label>
              <input type="text"
                     class="meta_image_url"
                     name="gallery[image_url][]"
                     value="<?php esc_html_e( $gallery_data['image_url'][$i] ); ?>"
              />
            </div>
          </div>

          <div class="field_right image_wrap">
            <img src="<?php esc_html_e( $gallery_data['image_url'][$i] ); ?>" height="48" width="48" />
          </div>

          <div class="field_right">
            <input class="button" type="button" value="Choose File" onclick="add_image(this)" /><br />
            <input class="button" type="button" value="Remove" onclick="remove_field(this)" />
          </div>

          <div class="clear" /></div> 
        </div>
        <?php
        } // endif
    } // endforeach
    ?>
    </div>

    <div style="display:none" id="master-row">
    <div class="field_row">
        <div class="field_left">
            <div class="form_field">
                <label>Image URL</label>
                <input class="meta_image_url" value="" type="text" name="gallery[image_url][]" />
            </div>
        </div>
        <div class="field_right image_wrap">
        </div> 
        <div class="field_right"> 
            <input type="button" class="button" value="Choose File" onclick="add_image(this)" />
            <br />
            <input class="button" type="button" value="Remove" onclick="remove_field(this)" /> 
        </div>
        <div class="clear"></div>
    </div>
    </div>

    <div id="add_field_row">
      <input class="button" type="button" value="Add Field" onclick="add_field_row();" />
    </div>

</div>

  <?php
}

/**
 * Print styles and scripts
 */
function print_scripts_so_14445904()
{
    // Check for correct post_type
    global $post;
    if( 'page' != $post->post_type )// here you can set post type name
        return;
    ?>  
    <style type="text/css">
      .field_left {
        float:left;
      }

      .field_right {
        float:left;
        margin-left:10px;
      }

      .clear {
        clear:both;
      }

      #dynamic_form {
        width:580px;
      }

      #dynamic_form input[type=text] {
        width:300px;
      }

      #dynamic_form .field_row {
        border:1px solid #999;
        margin-bottom:10px;
        padding:10px;
      }

      #dynamic_form label {
        padding:0 6px;
      }
    </style>

    <script type="text/javascript">
        function add_image(obj) {
            var parent=jQuery(obj).parent().parent('div.field_row');
            var inputField = jQuery(parent).find("input.meta_image_url");

            tb_show('', 'media-upload.php?TB_iframe=true');

            window.send_to_editor = function(html) {
                var url = jQuery(html).find('img').attr('src');
                inputField.val(url);
                jQuery(parent)
                .find("div.image_wrap")
                .html('<img src="'+url+'" height="48" width="48" />');

                // inputField.closest('p').prev('.awdMetaImage').html('<img height=120 width=120 src="'+url+'"/><p>URL: '+ url + '</p>'); 

                tb_remove();
            };

            return false;  
        }

        function remove_field(obj) {
            var parent=jQuery(obj).parent().parent();
            //console.log(parent)
            parent.remove();
        }

        function add_field_row() {
            var row = jQuery('#master-row').html();
            jQuery(row).appendTo('#field_wrap');
        }
    </script>
    <?php
}

/**
 * Save post action, process fields
 */
function update_post_gallery_so_14445904( $post_id, $post_object ) 
{
    // Doing revision, exit earlier **can be removed**
    if ( defined( 'DOING_AUTOSAVE' ) && DOING_AUTOSAVE )  
        return;

    // Doing revision, exit earlier
    if ( 'revision' == $post_object->post_type )
        return;

    // Verify authenticity
    if ( !wp_verify_nonce( $_POST['noncename_so_14445904'], plugin_basename( __FILE__ ) ) )
        return;

    // Correct post type
    if ( 'page' != $_POST['post_type'] ) // here you can set post type name
        return;

    if ( $_POST['gallery'] ) 
    {
        // Build array for saving post meta
        $gallery_data = array();
        for ($i = 0; $i < count( $_POST['gallery']['image_url'] ); $i++ ) 
        {
            if ( '' != $_POST['gallery']['image_url'][ $i ] ) 
            {
                $gallery_data['image_url'][]  = $_POST['gallery']['image_url'][ $i ];
            }
        }

        if ( $gallery_data ) 
            update_post_meta( $post_id, 'gallery_data', $gallery_data );
        else 
            delete_post_meta( $post_id, 'gallery_data' );
    } 
    // Nothing received, all fields are empty, delete option
    else 
    {
        delete_post_meta( $post_id, 'gallery_data' );
    }
}

Hi, here i implement multiple image upload custom field to page post type if you want it for another post type just change post type name.

Add this following code to your current theme’s function.php file. Go to your pages from wordpress admin and check that multiple image upload custom field is added to each page.

Wordpress add multiple image upload custom field to any post type

wordpress

PHP