BuddyPress.org

If you’re using a child theme of the BP default theme, you’ll have to alter the index.php and archive.php files in your template.

Thumbnails require a plugin or using WP’s post thumbnail feature:
http://markjaquith.wordpress.com/2009/12/23/new-in-wordpress-2-9-post-thumbnail-images/

The layout is more of a general issue. Look up grid based layouts on Google and fool around with floats in CSS to do what you want.