If you wanted to send a message to people on the boards and you wanted to convey the password as well, how would you do that?

Because the date and time are local to the user you can't use that because people are in different time zones.

You can't be SURE you know what the post number is because someone may make another post before you do.

You have the image file name.

You have your ID:

The simple way is to find your ID with 1 post in the clear and then use it for the password of the steganography used in your image before you post it.

Otherwise, you'd need a pre-shared key.