Images that contain hidden metadata, usually in the alpha channel. Can only be read from/written to by certain software or extensions. As of June 2023, nearly any website that retains alpha information does not wipe this data (*booru, Discord, Imgur, Twitter [under certain resolutions], etc).
This technique originated as an extension made for the Stable Diffusion web UI, but websites such as NovelAI have now began to implement this as part of their service. NovelAI is notable for using this, as the signature used to verify if an image contains metadata is identical to the one used as part of the web UI extension (something that is very easy to change and thus would have made it more difficult to detect).
