Secure Multiple WordPress Installations on shared hosting

It would be better to use file permissions. However, this doesn’t work with all hosters.

Go to your FTP explorer and change the file permissions of an image to 400 (only read for my own user, all others have no right at all). Can you still access this file via the web or do you see a 403 Forbidden error? If it works you can do this for all files and add write permissions where you need them. Then try out if the upload still works.

If the other sites have different file users, then this will prevent them from accessing your files. This normally requires CGI/FCGI or similar setups.