Great plugin! Here are a few quick feature requests to make it even better I think. Note that we are looking to use it in Multisite network settings, where each site admin has limited capabilities, and they're choosing from many available themes. (That might help explain/justify some of the ideas.)
1) "Thumb mode" should not even be an option if the current theme doesn't support Post Thumbnails. Otherwise, they're just dumping the same default icon all over. See http://codex.wordpress.org/Function_Reference/current_theme_supports
2) I believe that the default of 0 for number of posts (including Auto) is a bad choice, I believe that something like 3 or 4 is a much more reasonable default.
3) The settings page includes a php snippet. However, there are many times when the people doing the settings for this will not have the ability to put php on the site. I'd suggest only displaying this message if (current_user_can('edit_themes'))
4) I'm confused by the inclusion of "Shortcode mode" on the settings page without there even being a radio box to check it in the first place. If you're just trying to remind people that you can use this shortcode, I'd suggest just putting some text at the top instead.