Hey,
you can use post meta. I can give you some hints though:
1) You can use custom fields and the get_post_meta function: http://codex.wordpress.org/Function_Reference/get_post_meta to implement custom images. You need to modify the portfolio options if you want to use custom fields with portfolio entries. To do this, open /theme_options/portfolio.php and find this code
$args = array(
'labels' => $labels,
'public' => true,
'show_ui' => true,
'capability_type' => 'post',
'hierarchical' => false,
'rewrite' => array('slug'=>$slugRule,'with_front'=>true),
'query_var' => true,
'show_in_nav_menus'=> false,
'menu_position' => 5,
'supports' => array('title','thumbnail','excerpt','editor','comments')
);
Change the supports to include custom fields like this:
$args = array(
'labels' => $labels,
'public' => true,
'show_ui' => true,
'capability_type' => 'post',
'hierarchical' => false,
'rewrite' => array('slug'=>$slugRule,'with_front'=>true),
'query_var' => true,
'show_in_nav_menus'=> false,
'menu_position' => 5,
'supports' => array('title','thumbnail','excerpt','editor','comments','custom-fields')
);
2) Kriesi wrote a tutorial here: http://www.kriesi.at/archives/how-to-use-wordpress-custom-fields
I.e. use a custom field with the name "preview_image" and the value "http:/my-image.jpg". Depending on various decisions (i.e. do you want to set the fullwidth version by using post meta or the preview version, do you want to link the image or not, etc.) you need to implement the post meta code in your theme. You can use single.php (for the fullwidth version) or index.php + archive.php for the small image version. You'll need some php/html (maybe css) skills.
To receive the preview image url use:
<?php $image = get_post_meta($post->ID, 'preview_image', true); ?>