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To upload files you'll first need an account on Wikimedia Commons, and you have to be logged in.
If you already have an account on another Wikimedia project, for example at Wikipedia, then you should be logged in Commons automatically after logging in your home wiki. You also should be able to log in here using the same name and password. When not logged in to an account, by default a person edits instead by their IP address, assigned by their internet service provider (ISP). Nevertheless, creating an account is quick, free, and non-intrusive, and it is generally considered a good idea to do so for a variety of reasons.
Summary of benefits 
- Upload images
- Start new pages, including their own user page.
- Edit semi-protected pages once ten edits have been made, and their account is at least four days old.
- Send, and optionally receive, e-mails to and from other users.
- Customize the appearance and behavior of the website to their own preference.
- Keep a watchlist to track changes made on articles of interest.
- Log into other Wikimedia projects.
- Customize their own signatures when they sign on talk pages by going to their preferences.
Benefits explained 
If you create an account, you can pick a username provided it is available and unique. Edits you make while logged in will be assigned to that name. That means you will get full credit for your contributions in the page history (when not logged in, the edits are just assigned to your IP address). You can also view all your contributions by clicking the "My contributions" link, which is visible only when you are logged in.
You will have your own permanent user page where you can write a bit about yourself. While Wikimedia Commons is not a homepage provider, you can use this to display a few free pictures, write about your hobbies, etc. Many users use their user page to maintain a list of the pages they are most proud of, or to collect other valuable information from Wikimedia Commons.
You will have a permanent user talk page you can use to communicate with other users. You will be notified whenever someone writes a message on your talk page. If you choose to give an e-mail address, other users will be able to contact you by e-mail. This feature is anonymous; the user who emails you will not know your e-mail address.
Reputation and privacy 
You do not need to reveal your offline identity, but having an account gives you a fixed Wikimedia Commons identity that other users will recognize. While we welcome anonymous contributions, logging in under a pseudonym lets you build trust and respect through a history of good edits. It is also easier to communicate and collaborate with an editor if we know who you are (at least, who you are on Wikimedia Commons). It is also easier for veteran users to assume good faith from new users who take the effort to create an account (and you may well become a veteran user yourself some day!). You may well be afforded a great deal less leeway if you do not go to the trouble of making up a username.
You are actually more anonymous (though more pseudonymous) logged in than you are as an "anonymous" editor, owing to the hiding of your IP address. You might want to consider various factors, including privacy and the possibility of offline harassment, when selecting a username.
The privacy implications of this vary, depending on the nature of your Internet Service Provider, local laws and regulations, and the nature and quantity of your edits to Wikimedia Commons. Be aware that Wikimedia Commons technologies and policies may change.