theme content type with image from within paragraph
To get the url from an image field you have to go one level deeper, .entity gets the file entity where the URI of the image is stored:

{{ file_url(content.field_person[0]['#node'].field_image.0.entity.uri.value) }}


But digging through multiple levels of nested entities is not good practice, because this produces code hard to debug and maintain. You should look for a  better place to theme the image, for example node--person.html.twig, where you can print the image field, formatted how it is configured in UI:

{{ content.field_image }}