Looks like you're using the old version of the plugin with the faulty timthumb script. The plugin updates are displayed in WordPress only for plugins that are in the WordPress plugin directory, and since our plugins aren't free - they cannot be placed there.
To get your plugin secure again please grab the newest version of the timthumb file from here :
code.google.com/p/timthumb/
, rename it to the thumb.php and replace in all of the RT plugins on your site and in the theme directory too.
Thanks,
Jakub
Remember to always post a link to the site you're having problem with.
My ISP closed me down due to this same issue. The solution is to get the Tim Thumb Vulnerability Scanner plugin (
wordpress.org/extend/plugins/timthumb-vulnerability-scanner/
) and run it within Wordpress. It patched all the problem pieces of code for me and I am up and running again.