Jacinthe,
I can't teach you the entire structure of the Internet (in part because I don't understand it all myself!) but I can give you a few tips like I have been that should help with your own research. (Two of my favorite sources are Google & Wikipedia, both of which I think have substantial French functionality.)
URL = "Uniform resource locator" -- this is the generalized name of the specification the internet runs on. It is very powerful. The very first part specifies the protocol (like html = hypertext markup language) of the information contained in the rest of the URL, which is the
path to where it lives. It is an address, just like 111 Some Rue; Montreal, Canada, just written in a specific format computers can understand.
A browser accepts some (but not necessarily all) of the protocols & does something with them. For html, it reads the "marked up" text, which contains instructions (like the alignment tag that gave you problems) for how to display other content (like a gif image or text) in a "web page," which it creates according to those instructions.
The tricky part is that some of those instructions may tell the browser to fetch things to display from somewhere else, like the way you tell the browser in your posts to fetch an image, not from Apple Discussions or your computer, but from that French site full of gif images. That web site does the same thing: the web page you look at from that source is made up of (in part) instructions to fetch images from some specific URL (location or address) somewhere else.
So, if you paste the web site's address into the instructions to display your post, the browser would try to display the whole web page where you wanted to display a gif. Apple wisely limits what posts are allowed to support in the way of instructions, recognizes that the URL you referenced isn't a gif, & substitutes the "?" graphic instead.
But if you use the URL of a gif itself, as long as it meets certain tests (I don't know what they are, so don't ask), Discussions happliy will display it.
From here on, you are pretty much on your own. Use Google or similar to search for more in-depth discussions or tutorials. It is a huge subject but there are primers for beginners through very advanced tutorials available online. This should get you started & give you an idea of what to search for next.