Click or double click on the selected photo to be taken to the Flickr website. Open the actions menu, then click the '"View all Sizes" link inside the menu. On the next page, choose the size you want to download and click the link in the "Download" section. Please bear in mind, you may not have permission to download the original file so you will not see "View all Sizes" in the actions menu or the "Download" section on the "All sizes" page of every photo.