Doesn't sound like this is the right plugin for you. It has its usefulness for a large number of people, but it's definitely not something for everyone.
This is not a plugin, a plugin supposed to work as opposed to looking for files and inserting code into them according to the description of this "plugin".
Actually, that's not the definition of a plugin at all. A plugin is simply a PHP script that extends WordPress in some way. How it's used has no bearing on its status as a plugin.
...not to mention that it is useless the featured image inside wordpress does all what this "plugin was supposed to do".
This is also incorrect. The WordPress featured image functionality only does one thing this plugin does. Their purposes are the same, but what they can do are very much different.