When I click reply to a post like now, I get a section below the text I write called "Additional options" and a subsection "Attach files". Here I click on "Manage attachment" that handles the upload in a separate window. Then I close the window, and now below I see a name of my uploaded diagram as a link below. Then I right click on this name and chose "copy link". This link is the QB link adress to the uploaded png file and can be used as link reference.
I have not found an other way to insert the diagram in the text. When I just click on the icon or the name the diagram is not inserted by that. How did you avoid the file to be shown as a thumbnail below the uploaded text?