When images or other attachments are uploaded to a non-private WordPress site, the images are immediately public and can be shared on the Internet. This is why Contributors cannot normally upload images or files.
The VIP plugin repository file
vip-helper-wpcom.php includes a helper function called
vip_contrib_add_upload_cap() which will allow Contributors to upload. For Enterprise sites, there’s a plugin that can be activated from the admin called “Allow Contributors to Upload.”