Creating "open access" happens in three steps.
1: Create a "Guest" group. Use the group to set the desired permissions for guest access (remove ability to upload, in your case)
2: Create a "guest" user and put them in the "Guest" group.
3: Activate the anonymous login option in your config.php (see below)
# Uncomment and set the next line to allow anonymous access. The anonymous user will automatically be logged in
# to the account with the username specified.
# Note that collections will be shared among all anonymous users - it's therefore usually best to turn off all collections functionality for the anonymous user.
$anonymous_login="guest";
It's important to keep the guest user and $anonymous_login exactly the same. I'd also recommend sticking to the suggested "guest" user.
Let me know if you run into any other issues.