1) Get the URL of the image, or upload your own image
You can usually find the URL of an image by right clicking on it and choosing copy image address, or open image in new tab and it will be displayed in the address bar.
Alternatively, you can upload your own image to https://postimages.org and then copy the 'Direct Link' URL.
2) Paste the URL in to your discussion post
That's it! When you submit your post it will automatically convert your url text in to an image, just like this: