An image that has been converted from a lossy image format to a lossless image format. Usually this is a PNG image ({{filetype:png}}) full of JPEG Artifacts, because it was originally a JPEG.
Lossy-lossless images should not be uploaded to Danbooru when the original is available. It is Danbooru policy to delete third-party lossy-lossless images when the original is available. See Third-Party Edit.