I have encountered problems with TimThumb.php last days. Images have been succesfully uploaded, but timthumb.php...okey, you know what I mean. No thumbnails on your site. How did I fix it?
find CACHE folder in your Theme and change CHMOD of index.html & index.php in cache folder to 755. This is what fix my problem with TimThumb.php. (If nothing happens, change it to 777, it works 100%).