Different Featured Images depending on context: loop or single view

The easiest way to achieve this is to use the Multiple Post Thumbnails plugin. Once you have done this, add the following snippet to your functions file:

if (class_exists('MultiPostThumbnails')) {
    new MultiPostThumbnails(array(
    'label' => __('Secondary Featured Image', 'yourtextdomain')
    'id' => 'secondary-image',
    'post_type' => 'post'
    ));
    }

This will make a box ‘Secondary Featured Image’ available on your edit post page. Now you will have to place it in your theme, wherever you want the secondary featured image to appear in stead of the primary:

if (class_exists('MultiPostThumbnails')) 
    MultiPostThumbnails::the_post_thumbnail(get_post_type(), 'secondary-image');
else
    the_post_thumbnail(); //fallback in case somebody doesn't have the plugin installed