Hello nicolecoscio,
Its good to know your previous issues solved.
This plugin do not mess up your image, its only work is to crop images to predefined sizes http://i2.wp.com/www.doc.acmethemes.com/wp-content/uploads/2016/01/media.png.
If you afraid to do that in live site, you can try in local or dev site.
Best Regards
AcmeThemes