Your plugin references "thimbthumb.php" as a script it is looking for, however, the name of the script is TimThumb, and the file name the script may be in can vary from theme to theme and plugin to plugin.
As a result your plugin does not find the script in many cases.
What I would recommend is that you look for the contents of the script rather than the filename (which you have misspelled). The http://wordpress.org/plugins/timthumb-vulnerability-scanner/ plugin is a good example to follow for this. It is able to find TimThumb scripts which your plugin does not find.
Also, you do not need to delete the TimThumb script which would end up breaking your site if your theme depends on it. Instead, you should keep the script updated until you can install a theme or plugin that does not rely on it.
Otherwise, your plugin is very helpful. I love that it is lightweight and takes care of many basic security things that the more "full-featured" plugins like the iThemes security plugin doesn't address, such as simple file permissions check and fix.