This is more a WordPress issue than a Pi one to be honest but:
1) The WordPress application runs under the www-data user no matter who you sign on to the Pi or WordPress. This is the default web server user for Raspbian and should not be mixed up with your WordPress user. The WordPress user only controls the abilities within the application, such as edit, post, delete etc.
2) Depending on your install instructions you may not have loaded an editor, the WordPress recommendation is the PHP module imagick to be installed.
3) Using 777 in a security change is very very risky - you are basically giving any user full access to an object
Not the best security