I think it would be better if there was no way to tell whether or not a message has been hidden in an image. I guess what the code does now is record certain start and end markers so it is possible to know where the encoded material begins and ends. But how about this:

1. Make the encoded data start at the very beginning of the pixel data, or at some fixed location after that with no start marker, and place no marker saying where it ends. Needless to say, whatever original noise has not been overwritten should stay intact.
2. The decode command thus extracts whatever data has been hidden, and after it ends it returns the original noise of the picture. It is up to the user to decide where the hidden data ends.
   The reason for all this is that real steganography is undetectable. If the code adds detectable markers of any kind, the cover is blown.

It would be awesome to describe the algorithm (or its properties) a bit in the readme!

In the JSON example I added (see pull request), I tried to pass option t:2 or t:1 but the encoding (or decoding) fails.

I recently downloaded and converted steganography.js to be an es6 module.
You can see it here:
https://github.com/backspaces/agentscript/blob/master/vendor/steganography.js#L1

I don't know if you are interested in using this sort of thing. I used a simple imagePromise utility import below

export function imagePromise(url) {
    return new Promise((resolve, reject) => {
        const img = new Image()
        img.crossOrigin = 'Anonymous'
        img.onload = () => resolve(img)
        img.onerror = () => reject(`Could not load image ${url}`)
        img.src = url
    })
}

You could remove the dependency, replacing it with that code, or similar.

This isn't just es6 conversion, it corrects a problem with async vs sync image loading:

        loadImg: async function (url) {
            var image = await asutils.imagePromise(url)
            return image
        },

.. which in turn requires async/await which are only available in modules, sigh!

Anyway, if you'd like help with any of this, just ask.