If you haven't already done so you might like to read the previous post on Creative Commons licenses. CC licenses are the things that make it possible to find photos and images to use for free on Flickr, but there are usually still terms and restrictions, so it is important to understand what is being offered. Usually attribution is required; so you need to credit the photographer as author of the work.
The whole of Flickr is not a source of free photos! While Flickr is the Internets biggest photo sharing site the term 'showing' would really be more accurate. Photo sharing on Flickr does not mean any image can be downloaded and used for free. It depends on the license a Flickr user has chosen to publish an image with and if they have gone with traditional copyright "all rights reserved" then the image may not be used without first seeking permission to do so.
However Flickr is a great source for free photos and images if you know where to look! To find photos for free we need to filter Flickr's collection to show us only images available under Creative Commons licenses. Before starting a specific search you should be clear about what you need the photo for in order to look for the applicable CC license that will allow this.
To search for CC photos
Go to the advanced search page on Flickr. This page gives you various options for fine tuning a search on Flickr. The last of these at the bottom of the page is an option to show only content with a creative commons license. This is what you want to find images, and videos, available to use for free. If you want photos etc to use commercially or to adapt yuo can further filter the results to allow for this.
To browse for CC photos
Or to use Flickr terminology, Explore! Go to the Creative Commons page on Flickr. Here you'll find brief definitions of the different license options and links to explore content available under each diferent set of options.
Flickr can be a gold mine for creative photos and a lot of great content is available to use for free. Just remember to credit authors and abide by any other CC terms and if you find an image that is just what you need but isn't Creative Commons licensed then be sure to contact the photographer to seek permission to use their work.