In reply to Marc C:
When you view the photos in the 'lightbox' there should be an option (top right) to view more sizes. This will display all of the available sizes and you just need to click the largest one, followed by a right click and save.
I'm assuming it's not your Flickr account and thus you might find the user has disabled downloading of their photos, which might prevent the above.
Remember to observe copyright and check with the photographer that they are happy for you to use their photos.
The other thing to bear in mind is that if they are just 'web quality' i.e. 72 ppi that they might not of a sufficient quality to print. You should be able to check this in the exif data for the file.