If you're interested in posting photos online and want to make sure you get your social media presence known, you still need to understand how social media works. In many cases, you won't retain the sole right to your images.
When you post content on the internet, you own the copyright right away. As soon as the idea, photo, poem or other document is online, you have a timestamp and can claim it as your own. However, there's a problem. You can't usually stop the service from using your photos or information in whatever way it's allowed through the site's terms of service.
Why are a site's terms of service important?
The website's terms of service state whether or not the company will use or take photos or other content from you for its own personal use. In many cases, the site can use your information for promotional purposes or for whatever business-related purposes it has in mind. For example, on Facebook, signing the terms of service means you're giving the company the right to use your content free of charge.
Keep in mind that you never actually lose the copyright to your image or content when you post on these websites, but it does open it up to being shared and misused by others. If someone does take a photo or content from your page and uses it offline to make a profit or in other ways, then it's possible that you could pursue a claim against that person. After all, they don't have a copyright to use your work, the social media site does.
Be cautious, especially if you note "creative commons" in the terms of service. If you upload content through creative commons licensing, then you're giving permission to others to use your work within the terms of that agreement. Some make it legal to use it without mentioning the original author. Other times, your information has to be included and referenced any time the work is used off-site.