A very simple format would be regular fixed dot patterns (⣿) corresponding to bytes, spaced a apart a bit. Then it's the user's responsibility to be sensible and encode stuff without too clever compression. This wouldn't guard against trying to read the image sideways or upside down though.

If you get a disc with ASCII text and can read the bits a 100 years from now, even if ASCII coding is lost to history, as long as you can figure out the thing is supposed to contain writing that uses 8 bits per letter, and still know English, a trivial pattern analysis gets you the code table you can use to read the text.

The plaintext can then be used to describe the formats for whatever other data the disk may contain.