Still getting the hang of gists apparently

js_resize.php

<?php
/*
 * Resize images dynamically using wp built in functions
 * Based on the script by Victor Teixeira
 * - Updated to use wp_get_image_editor()
 * - Moves resized files to uploadpath/resized/
 * Joe Swann
 *
 * php 5.2+
 *
 * Usage example
 * 
 * <?php 
 * $thumb = get_post_thumbnail_id(); 
 * $image = js_resize( $thumb, '', 140, 110, true, '/resize_folder/' );
 * ?>
 * <img src="<?php echo $image[url]; ?>" width="<?php echo $image[width]; ?>" height="<?php echo $image[height]; ?>" />
 *
 * @param int $attach_id
 * @param string $img_url
 * @param int $width
 * @param int $height
 * @param bool $crop
 * @param string $resize_folder
 * @return array
 */
if ( !function_exists( 'js_resize') ) {
	function js_resize( $attach_id = null, $img_url = null, $width, $height, $crop = false , $resize_folder = '/resized/') {

		// this is an attachment, so we have the ID
		if ( $attach_id ) {

			$image_src = wp_get_attachment_image_src( $attach_id, 'full' );
			$file_path = get_attached_file( $attach_id );
		
			$orig_url_path = dirname(wp_get_attachment_url( $attach_id ));

		// this is not an attachment, let's use the image url
		} else if ( $img_url ) {

			$file_path = parse_url( $img_url );
			$file_path = $_SERVER['DOCUMENT_ROOT'] . $file_path['path'];

			// Look for Multisite Path
			if(file_exists($file_path) === false){
				global $blog_id;
				$file_path = parse_url( $img_url );
				if (preg_match("/files/", $file_path['path'])) {
					$path = explode('/',$file_path['path']);
					foreach($path as $k=>$v){
						if($v == 'files'){
							$path[$k-1] = 'wp-content/blogs.dir/'.$blog_id;
						}
					}
					$path = implode('/',$path);
				}
				$file_path = $_SERVER['DOCUMENT_ROOT'].$path;
			}
			//$file_path = ltrim( $file_path['path'], '/' );
			//$file_path = rtrim( ABSPATH, '/' ).$file_path['path'];

			$orig_size = getimagesize( $file_path );

			$image_src[0] = $img_url;
			$image_src[1] = $orig_size[0];
			$image_src[2] = $orig_size[1];
		
			$orig_url_path = dirname($image_src[0]);
		}

		$file_info = pathinfo( $file_path );

		// check if file exists
		$base_file = $file_info['dirname'].'/'.$file_info['filename'].'.'.$file_info['extension'];
		if ( !file_exists($base_file) )
		 return;

		$extension = '.'. $file_info['extension'];

		// the image path without the extension
		$no_ext_path = $file_info['dirname'].$resize_folder.$file_info['filename'];

		$cropped_img_path = $no_ext_path.'-'.$width.'x'.$height.$extension;

		// checking if the file size is larger than the target size
		// if it is smaller or the same size, stop right here and return
		if ( $image_src[1] > $width ) {

			// the file is larger, check if the resized version already exists (for $crop = true but will also work for $crop = false if the sizes match)
			if ( file_exists( $cropped_img_path ) ) {

				$cropped_img_url =  $orig_url_path . $resize_folder . basename( $cropped_img_path );

				$js_image = array (
					'url' => $cropped_img_url,
					'width' => $width,
					'height' => $height
				);

				return $js_image;
			}

			// $crop = false or no height set
			if ( $crop == false OR !$height ) {

				// calculate the size proportionaly
				$proportional_size = wp_constrain_dimensions( $image_src[1], $image_src[2], $width, $height );
				$resized_img_path = $no_ext_path.'-'.$proportional_size[0].'x'.$proportional_size[1].$extension;

				// checking if the file already exists
				if ( file_exists( $resized_img_path ) ) {

					$resized_img_url =  $orig_url_path . $resize_folder . basename( $resized_img_path );

					$js_image = array (
						'url' => $resized_img_url,
						'width' => $proportional_size[0],
						'height' => $proportional_size[1]
					);

					return $js_image;
				}
			}

			// check if image width is smaller than set width
			$img_size = getimagesize( $file_path );
			if ( $img_size[0] <= $width ) $width = $img_size[0];

			// no cache files - let's finally resize it
			//$new_img_path = image_resize( $file_path, $width, $height, $crop, NULL, NULL, 100 );
			//we want to use a custom folder for resized images
			$new_dir = $file_info['dirname'].$resize_folder;
			if (!file_exists($new_dir)) {
			    mkdir($new_dir);
			}

			$editor = wp_get_image_editor( $file_path );
			if ( is_wp_error( $editor ) )
			    return $editor;
			$editor->set_quality( 100 );
			$resized = $editor->resize( $width, $height, $crop );

			$dest_file = $editor->generate_filename( NULL, $new_dir );
			$saved = $editor->save( $dest_file );
			if ( is_wp_error( $saved ) )
			    return $saved;
			$new_img_path=$dest_file;
			$new_img_size = getimagesize( $new_img_path );
			$new_img =  $orig_url_path . $resize_folder . basename( $new_img_path );		

			// resized output
			$js_image = array (
				'url' => $new_img,
				'width' => $new_img_size[0],
				'height' => $new_img_size[1]
			);

			return $js_image;
		}

		// default output - without resizing
		$js_image = array (
			'url' => $image_src[0],
			'width' => $width,
			'height' => $height
		);

		return $js_image;
	}
}

Includes the updated wp_get_image_editor() update (below) and moves resized images to a subfolder to aid with clean up of unneeded image sizes in large installations during development

http://www.munto.net/blog/2013/01/22/vt_resize-php-image_resize-deprecated/