Anyone posting an image must post only their own images or the images of someone who has given them permission to post them, which often includes giving appropriate credit to the original photographer. Photos from publications or downloaded from elsewhere on the web are not allowed.
When uploading photos to Flickr, you can give several different levels of permission for others to use your work, including All Rights Reserved and Creative Commons licenses.
You don't have to make the photos you upload to Flickr visible to the public. They can be visible only to you or to other members designated as friends or family. You can share nonpublic photos from your Flickr account with nonmembers via a "guest pass." You can also designate who is allowed to comment on your photos or tag them.