Currently images cannot be uploaded to the forum due to some plugin issues. I will try to resolve the issue ASAP.
Workaround: Upload images to third-party image hosting site (e.g., Imgur -> https://imgur.com/) and select "Image Link" icon from the menu bar when writing or replying to a post. You will get a template in the following format:
![alt text](image url)
Just replace the "image url" with the url of the uploaded image. The image should be displayed in the preview window.
Optionally specify "alt text".
Sorry for the inconvenience.