Simple Featured Posts is a pratical widget that allows you to show a post list with thumbnails ordered by random or recent posts.
You can exclude categories using "-" before its ID.
You can replace default image in '/wp-content/plugins/simple-featured-posts-widget/images/' folder.
Download latest version of TimThumb, placed in a folder on your website. In the same directory as the timthumb.php file you need to add 1 directory. The directory should be called 'cache' and needs to have its file permissions set to 775.