Auto thumbnail resizing?

Hello! This is my first post here, i hope everything will be ok. 🙂

Currently i have a theme where thumbnails are loaded my making a custom tag “thumbnail” and adding the url on the value. It’ll automatically resize the images to fit the box. But they get distorted. Are there any plugins that can crop the images so they don’t distort, and then give me an url so i can paste it into the value box?

Thanks!
Andrei