Automatically display a default Post Thumbnail image, either the first image attached to a post, or set default images for specific categories, tags, or custom taxonomies. Supports custom fields to ease upgrading.

Download: http://wordpress.org/extend/plugins/default-thumbnail-plus

Thumbnail Selection Hierarchy

The hierarchy below determines what thumbnail will be chosen, in other words the plugin first checks if the user has manually set a featured image. If not then it checks if it has a custom field containing an attachment id, and so on. Once the plugin gets to the last step if no default thumbnail image is set then no post thumbnail will be displayed.

1. Featured Image
2. Custom Field
3. Image Attachment
4. Embedded image
5. Embedded video
6. Category/Tag/Taxonomy Thumbnail
7. Default Thumbnail

Add Filter (Category/Tag/Taxonomy Thumbnail)

You can add filters which allow you to select a thumbnail for a specific category, tag, or custom taxonomy. The value field is required and must contain either a comma separated list of slug, id, or title for a existing category, tag, or custom taxonomy. Any post that does not have a post-thumbnail set and has the specified tag, or is a member of the category or taxonomy will show the default thumbnail specified.

Use image attachment if available

Automatically use the post’s first available image attachment for the thumbnail. This is useful for older posts that haven’t got a featured image set.

Custom Field

Enter a custom field key here, it’s value if set will become the default post thumbnail for that post. The custom field value can either be an Attachment ID, or a link to an image.

Note: If the custom field value contains a link to an image it will output a simple image tag and attempt to populate the width and height attributes. However the aspect ratio may not match other thumbnails. The plugin does not check if the image exists or is of valid type or size.

Use embedded image if available

Automatically use the post’s first available embedded image for the thumbnail. This is useful if you embed external images.

Note: This option does not work with has_post_thumbnail() or get_post_thumbnail_id(). If the “cache external images” option is set then the plugin will automatically resize, crop the image and store a local copy.

Use embedded video if available

Automatically use the post’s first available embedded video for the thumbnail (YouTube only).

Note: This option does not work with has_post_thumbnail() or get_post_thumbnail_id(). If the “cache external images” option is set then the plugin will automatically resize, crop the image and store a local copy.

Excluded posts

List of posts to be ignored by this plugin. Comma separated e.g. 10, 2, 7, 14.

Note: These posts may still display a thumbnail if one has been set manually (using the “Set featured image” button on the post page).

Functions

The following functions work to directly return post thumbnail even if the plugin hooks have been disabled. This allows you to disable the filters and call the plugin manually. This should only be used if you are creating a custom theme and can ensure that DefaultThumbnailPlus is installed.

dpt_post_thumbnail_id($post_id, $size = null, $attr = ”)
Returns the post thumbnail ID. Doesn’t work with embedded images.

dpt_post_thumbnail_html($post_id, $size = null, $attr = ”)
Directly calls the default post thumbnail function. Works the same as calling get_the_post_thumbnail().

dpt_post_thumbnail_src($post_id, $size = null)
Returns href to post thumbnail image.

Why can’t I see any thumbnails with my custom theme?

A number of themes use has_post_thumbnail() to check if a post thumbnail exists before calling the_post_thumbnail() to display the thumbnail. The problem with this is that thumbnails that are linked to directly (either external images or video thumbnails) won’t return correctly for this check and therefore may not show.
 
To fix this issue you will need to look for the following code in your theme:

if(has_post_thumbnail()) {
    the_post_thumbnail(...);
}

 
And simply remove the call to has_post_thumbnail() so you are left with this:

the_post_thumbnail(...);

TwentyEleven

The header only works with image attachments, i.e. embedded images/video thumbnails won’t show.

The header attachment width also has to be greater than the custom-header width option (default 1000px). This is generally a good thing though since it prevents low resolution images from displaying in the header.

WooThemes

WooThemes call a function called woo_image() rather than using the default WordPress functionality. This causes the default post thumbnail to never show.

So to fix this you need to override the woo_image() function with a custom implementation that reverts to using WordPress’s in-built functions. To do this paste the following code into the TOP of the themes functions.php file found in /wp-content/themes/your_themes_name after the opening <?php tag.

//Override the default woothemes image function and revert to using the default wordpress function.
if ( ! is_admin() && ! function_exists('woo_image')){
    function woo_image($args) {
        global $post;

        $size = null;
        $attr = array();
        $before = '';
        $after = '';
        $class = '';

        if ( !is_array($args) )
            parse_str( $args, $args );

        extract($args);

        if(empty($id))
            $id = $post->ID;

        if(isset($width) && isset($height))
            $size = array($width, $height);

        $attr['class'] = 'woo-image '.$class;

        if(isset($alt))
            $attr['alt'] = $alt;

        $output = $before . get_the_post_thumbnail($id, $size, $attr) . $after;

        if($return == TRUE)
            return $output;
        else
            echo $output;
    }
}

 
The downside is that it won’t recreate all of woo_image()’s features. However it shouldn’t cause issues with most themes.