The “All SIZES” button takes you to a page that shows the photo in a couple of different dimensions. You can then download the Photo in the size you want. Many Flickr users allow others to use their pictures under certain conditions.
Just below the “tags”on the right hand side of the picture you will find a section called “Additional Information”. If you see a text that says “All rights reserved” then it is better to contact the Flickr user before using his or her Photos.
If you see “Some rights reserved” then click on this link to see the conditions under which you can use the image. The link takes you to a page on the Creative Commons website. Creative Commons an 'open content licensing' project that allows you to specify to what degree your images (or other works) may be shared or used. If you are using images to promote an issue it's a good way to encourage people to pass your work on and get your message out there far and wide.