Manga-mode images on pixiv come in two main sizes: "sample" and "original". The sample sizes (e.g. http://i1.pixiv.net/c/1200x1200/img-master/img/2014/09/30/21/22/44/1498651_p0_master1200.jpg) are shown on the mode=manga page; they are always JPEG and have a maximum width and height of 1200px. The original sizes (e.g. http://i1.pixiv.net/img-original/img/2014/09/30/21/22/44/1498651_p0.jpg) are shown on the mode=manga_big pages, accessible by a small icon next to each page if you are logged onto pixiv.
The sample and original sizes could be exactly identical, barely different, or significantly different. This tag applies to any post whose MD5 hash matches the sample size and not also the original size. Do not add this tag unless you can confirm a difference, or remove it unless the hashes match.
If the image is of acceptable quality, upload the original size, make it the parent of the sample version, and flag the sample for deletion. It is Danbooru policy to delete these samples whenever the full size is also on Danbooru.
See topic #5200 for more discussion on this development.