An image containing text or a logo superimposed on top of the image. Typically to identify the owner/creator of the work and/or prevent the image from being counterfeited. Some watermarks are also deliberately placed as a "paywall" on "samples" of "paid reward" images.
Posts with watermarks added by third-parties (anyone other than the artist) should not be uploaded.
Add watermark grid if it spans over a majority/the whole image. Add too many watermarks if it's in excess as to be noticeable from the thumbnail.
Pixiv
Twitter
Bluesky
Weibo
Lofter
Tumblr
