It has been enabled a service to protect the site from "bandwidth stealing" to avoid unwelcome links to your material.
If the webmaster wants to enable the direct linking for some images, he has to place them in a directory called _altervista_ht placed in the root of the web space. Any other directory do not allow the direct linking.
Using an account exclusively or primarily as storage for files used by other sites is not allowed by AlterVista's regulation.