These functions can be added to Matt Varone's widget helper class (https://github.com/sksmatt/WordPress-Widgets-Helper-Class) to add a Media Upload field to a WordPress widget

widget-helper.php

/**
 * Load Media Upload Scripts
 *
 * Load the JS & CSS needed for the media upload field
 */
function enqueue_scripts() {
		
	// Only load on widgets screen
	$screen = get_current_screen();
	if ( $screen->id != 'widgets' )
		return;
	
	// Register media upload JS file
	$js_file = get_stylesheet_directory_uri() .'/js/media-upload.min.js';
	wp_register_script( 'wptuts-upload', $js_file, array('jquery','media-upload','thickbox') );
				
	// jQuery
	wp_enqueue_script('jquery');

	// Thickbox
	wp_enqueue_script('thickbox');
	wp_enqueue_style('thickbox');
	
	// Uploader scripts
	wp_enqueue_script('media-upload');
	wp_enqueue_script('wptuts-upload');
}


/**
 * Media Upload Field
 *
 * Allows you to add a WordPress thickbox media upload popup field
 * to a given widget
 *
 * @param $key array() Settings for the given field
 * @param $out string Anything to display before the field
 * @return $out HTML output to display the field
 */
function create_field_media_upload( $key, $out = "" ) 
{
	// Create label
	$out .= $this->create_field_label( $key['name'], $key['_id'] ) . '<br/>';
	
	// Created input text field
	$out .= '<input type="text" ';
	
	// Add class
	if ( isset( $key['class'] ) )
		$out .= 'class="' . esc_attr( $key['class'] ) . '" ';
	
	// Add ID and value attributes
	$value = isset( $key['value'] ) ? $key['value'] : $key['std'];
	$out .= 'id="' . esc_attr( $key['_id'] ) . '" name="' . esc_attr( $key['_name'] ) . '" value="' . esc_url( $value ) . '" ';
	
	// Input size
	if ( isset( $key['size'] ) )
		$out .= 'size="' . esc_attr( $key['size'] ) . '" ';             
	
	$out .= 'style="display:inline-block; width:161px" />';
	
	// Upload button
	$out .= ' <input id="btn-' . esc_attr( $key['_id'] ) . '" type="button" class="media-upload-button button" value="Upload" style="font-size:12px; display:inline-block;" />';
	
	// Description
	if ( isset( $key['desc'] ) )
        $out .= '<br/><small class="description">'.esc_html( $key['desc'] ).'</small>';
	
	return $out;
}

These functions can be added to Matt Varone's widget helper class (https://github.com/sksmatt/WordPress-Widgets-Helper-Class) to add a Media Upload field to a WordPress widget