Once the image is in the Gallery, you can add an image to your post by copying the URL that is in the box labeled "Image URL/Location:" on the photo's page. In your Forum post, click the "Image" button and paste the URL into the box.
Upload image
Copy URL from box
Paste URL into Insert Image dialog in Forum
That should do it! I copied the URL given on your bus picture and am showing it below. I usually change the size to no bigger than 500 wide for forum posts so it doesn't make them crazy big :)