I'm using phpThumb - the PHP thumbnail generator.
'phpThumb.config.php':
$PHPTHUMB_CONFIG['cache_maxage'] = 10;
$PHPTHUMB_CONFIG['cache_maxsize'] = 1000;
$PHPTHUMB_CONFIG['cache_maxfiles'] = 10;
but it does nothing... I've got 108 MB in 922 files... and it keeps growing.
I'd check that the user that php is running as has write permissions on the cache folder.
Have you checked the php error log?