Featured imagine only for specific category

Hi faina,

Sorry I did not see this message before.
You can set a different image per category with the following code:

add_filter( 'dfi_thumbnail_id', 'dfi_category', 10, 2 );
function dfi_category( $dfi_id, $post_id ) {
    // Set a different image for posts that have the 'cats' category set.
    // This will trigger first, if multiple categories have been set.
    if ( has_category( 'cats', $post_id ) ) {
        return 7; // cats img id.
    }
    // Set a different image for posts that have the 'cats' category set.
    if ( has_category( 'dogs', $post_id ) ) {
        return 8; // dogs img id.
    }

    return $dfi_id; // the original featured image id.
}

Let me know if you need help in adding this.

Jan-Willem