Your selfies are your property.
However most social media platforms have a user agreement requiring you to sign permission for reuse over to the owning company. This is why Facebook is allowed to sell and redistribute your images even though they don't own the copyright.
But what if a stranger downloads your image and posts it to a blog without your permission? This is illegal, but it’s very difficult to prevent.
The internet is a sea of illegally reproduced images. Use caution when reproducing images from the internet in slide presentations and research projects. If you can’t identify the original creator, the image may not be legally reproduced. When in doubt, consult with a librarian.
Image: Ibrahim Husain Meraj [CC BY-SA 3.0 (https://creativecommons.org/licenses/by-sa/3.0)], from Wikimedia Commons.
Want more information? Take a deep dive on copyright with DCCC’s Copyright Tutorial. It goes into more detail, and it covers many topics not mentioned here, including the Digital Millennium Copyright Act, Creative Commons and open access content, registering your copyright, and much more.