@Lieux here's how I get a screenshot of my computer screen and put it in a post here. No warranty this is the most efficient way to do it, but hey... it works !
You hit the "print screen" key on the keyboard (mine says "PrtSc"), this puts an image of your screen in the clipboard.
Then I open Paint3D, a Microsoft tool I think (maybe it replaces the old "Paint" ?) and paste the clipboard into it.
You can use the cropping tool to keep only what is relevant and then save it.
You are then ready to type you message. When you want to insert the image, you click on the "Photos" icon, then "Upload", "Choose files" and the rest is easy I think.
I hope this helps. If anyone knows a better way, please let @Lieux know about it (and I'll learn as well ! 
)
Here is what you should see:
