I know it's been a while but I finally figured out how to get the WhatsApp notification image.

I decompiled an app that could intercept the images (it's on the Playstore) and I managed to find out how it did it.

First get the URI of the image:

private fun getUriFromNotification(notification: Notification): Uri? {
    val parcelableArray = notification.extras.getParcelableArray(m.EXTRA_MESSAGES)
    return parcelableArray?.let { getUriFromParcelableArray(it) }
}

private fun getUriFromParcelableArray(parcelableArr: Array<Parcelable>): Uri? {
    if (parcelableArr.isEmpty()) return null

    val lastParcelable = parcelableArr.last()
    return if (lastParcelable is Bundle) {
        lastParcelable.getParcelable("uri")
    } else {
        null
    }
}

After that convert to your taste (mine is base64)

fun getFileAsBase64(context: Context, uri: Uri): String? {
        val contentResolver: ContentResolver = context.contentResolver
        val inputStream: InputStream? = contentResolver.openInputStream(uri)

        inputStream?.let {
            // Converte InputStream para Base64
            val byteArrayOutputStream = ByteArrayOutputStream()
            val buffer = ByteArray(1024)
            var bytesRead: Int
            while (it.read(buffer).also { bytesRead = it } != -1) {
                byteArrayOutputStream.write(buffer, 0, bytesRead)
            }
            val byteArray = byteArrayOutputStream.toByteArray()

            // Converte para Base64
            return Base64.encodeToString(byteArray, Base64.NO_WRAP)
        }

        return null
    }