Have you looked at using add_image_size? Check it out here.
It will allow you to crop images various ways. It will also allow you to use the standard the_post_thumbnail and pass it your custom image size when you go to output this on the front-end.
I also just noticed that you said inside the content. One way would be to take the above function and wrap it into a shortcode. Then you could pass the shortcode the needed arguments to output the correct size image.