When contrib images are missing in drupal the cause more often than not is that you have permissions problems with regards to your filesystem.
Make sure that your webserver can write towebserver can write to your files and files/styles directories.files and files/styles directories.
If permissions are ok then drupal will create your medium_large directory and subdirectories.
Check the drupal setup documentation here (see the paragraph "Create the files directory").
As mentioned there make sure that you have write permissions on your sites/default/files directory.