"These images were processed for inclusion on our websites, including adjusting color and brightness levels, and reducing the images in size to about 1000 dpi (dots per inch) for the high-resolution versions." Because there was so much demand for higher-resolution versions, Teague decided to reprocess the entire set and upload them to Flickr magazine by magazine.
What do they exactly mean by magazine by magazine? Magazines are weekly or monthly publications with articles and illustrations dedicated to a particular theme. Examples include Vibe, National Geographic etc. I don't think NASA stores their photo material in the form of a magazine.