You need to enter a valid image link between the image tags.
By "valid image link", I mean that it has to target an image, not a website, and it must be reachable from the internet.
Links that begin with file:/// are not networked and exist only on your system. You need to upload them somewhere.
This forum has an attachment system (Click on [Go Advanced] to see it, then go below the message edit field), the which you can use to upload whatever you need to showcase, and then attach them to the proper spot in your message with the paperclip button.
It will show a thumbnail to your image, but clicking it expands it to its original size.
If you need the image to begin as full width instead of being a tiny clickable thumbnail, you can use the attachment's link as your image link!
Or you can try uploading it to an image host (Imgur, to name one), but use it as your last option.