WordPress already does the heavy lifting for you on this. All you need to do is use post thumbnails. If you don’t see the featured image meta box on your pages/posts, place the following in your functions.php file.
add_theme_support('post-thumbnails');
From there you will need to define a new image size in your functions.php file.
add_image_size('special-image', 800, 600, true);
The fourth argument ‘true’ means WordPress will do a hard crop and slice the image to this exact dimension. You can get rid of the custom field as this is a baked-in feature. To use this in your theme you will just use this tag:
the_post_thumbnail('special-image');
//or
get_the_post_thumbnail($post->ID, 'special-image);
For the images that you’ve already uploaded, I would recommend you install the Regenerate Thumbnails plugin. http://wordpress.org/extend/plugins/regenerate-thumbnails/. This will go back to all the images that have been uploaded and apply your specific size.
To learn more about post thumbnails: http://codex.wordpress.org/Post_Thumbnails
Hope this helps!