The locks next to a category (like #your-photos) mean that it’s restricted to account-holders only, and is only visible if you’re logged in. Closed topics also get a padlock icon, but they show up next to the topic title in the main lists (Latest, Top, etc).
You can change your profile picture from your Preferences/Account page. 
There’s some other photo related stuff too, but you can find all the info in The Great Big Photo Guide 
(and the Welcome Topic has loads of good links in to all the other forum information/guides )