How to add an image to a thread post

Undoubtedly the answer to this question is obvious, but I am afraid I do not know the answer, somewhat a Luddite I am. I attempted to add an image to a recent post, but encountered an odd looking dialogue box after clicking on the image icon. It's preview pane was written in Latin, and there was some other gibberish concerning an "alt attribute." There did appear to be a URL field, but I am looking to insert an image inline, so that it is visible in the post, not by way of a link.

Please excuse the ignorance, always looking to learn, greatly appreciate any assistance with this.