Featured Images not appearing

All your issues could be solved with some MySQL queries.

Regarding point #1: if you are using mod_rewrite then I recommend using absolute URLs for images in posts, if you are displaying full post content (including its images) on other pages – as you suggest.

You can do a MySQL search/replace as follows:

UPDATE wp_posts SET post_content = replace(post_content, 'old string', 'new string');

So, for example, if all your images are located in the folder wp-content/uploads (that’s where WordPress usually places the media), then your search/replace could be something like:

UPDATE wp_posts SET post_content = replace(post_content, 'src="https://wordpress.stackexchange.com/questions/83310/uploads/", 'src="http://www.example.com/uploads/');

Regarding point #2:

You’re going to need to grab the ID of the image from wp_posts table, in order to add it to the wp_postsmeta table.

Let’s say you have in wp_postsmeta some meta data for post_id 1. One of these is ‘featured image’, so the row would be something like:

post_id = 1;
meta_key = 'featured_image';
meta_value="uploads/2012/12/someimage.jpg";

to get the ID of that image, you’d do:

SELECT ID FROM wp_posts WHERE guid LIKE '%uploads/2012/12/someimage.jpg';

You’d then be able to insert to wp_postsmeta the data as:

INSERT INTO wp_postsmeta (post_id, meta_key, meta_value) VALUES (1, '_thumbnail_id', ID-OF-IMAGE);

So, to combine it all, use a simple php script as follows:

    $posts = $wpdb->get_results("SELECT post_id, meta_value FROM wp_postmeta WHERE meta_key='featured_image'", ARRAY_N);
    if ($posts) {
        while ($posts as $post) {
            $image = $wpdb->get_row("SELECT ID FROM wp_posts WHERE guid LIKE '%" . $post[1] . "%'", ARRAY_N);
            if ($image_id) {
                $wpdb->query("INSERT INTO wp_postsmeta (post_id, meta_key, meta_value) 
                  VALUES ($post[0], '_thumbnail_id', '" . $image[0] . "')");
            }
        }
    }

Hope all of this gives you some direction and assistance. I’ve written it in broad terms, so please remember to back up your db before doing any changes!

Good luck.