You've only just discovered this stuff? 'just took a course or something?
In our modern (but more and more ancient) computerised world, this sort of things has been around for (a few...) years and there must be thousands of tuts and video recordings or referencing on the Internet about this, including about steghide! Example: https://www.maketech...mages-in-linux/
Under OS X/macOS a simple file concatenation line command will achieve this. Example as explained here:
you have a png picture file called Picture.png
you have a text file called Secret_stuff.txt
you can hide the text file inside a new picture file with Terminal command: cat Picture.png Secret_stuff.txt > NewPix.png
you will obtain what will look like a mirror copy of your original png picture file that you can open & display on screen through doubleclick
Here's a video tut from a few years ago for Windows which is very similar to yours (a simple binary mode copy command does the trick there).
As for steghide, 'not the freshest thing around either (no offense to its author meant)! e.g.: https://linuxfr.org/...ide-en-français from... 2002! Windows was XP and Mac OS X was Puma on Power Macintosh in those days.
In fact, most of the literature on steghide, including your post, doesn't entirely describe what it is and/or give credits to its author, Stefan Helz. The program might be open source and 15yr old but, nevertheless, it's not a built-in Linux Terminal command as kind of often hinted, probably because it's directly available as a downloadable package to most Linux distros (through apt-get install command)... Like many before you, you've listed and described the syntax of the steghide command without even referring to the (on-line) user manual it's literally copied from. C'est la vie...
I'm sure you meant well with your tutorial but you should always care to quote your sources, give due credits and not sail too late after the tide (15years is a tad late). Maybe teachers still use this in their classrooms.