How can I link mainpage slider to another page/post/external image?
you need to be comfortable with using wordpress custom fields. you need to create a new custom field, maybe call it “new_link” and then enter the link into this custom field. then you need to edit the template files.
in index php search for $link = get_permalink();add a new line afterwards:
$new_link = get_post_meta($post->ID, "new_link", true);
then search for get_permalink() arround line 47 and 52 and replace it with $new_link then search for $link at line 45 and replace it with $new_link as well