How do you get the url of an image in a view twig template?
There is a raw variable available - you can use it to get raw result from the query, with all data it fetched. 
You can inspect it with the devel or twig_vardumper module using xdebug or dump(raw). More about debugging twig templates: https://www.drupal.org/docs/8/theming/twig/debugging-twig-templates#s-viewing-variables