The image icon is irrelevant. The BB code is irrelevant. They both work as expected (outputting an tag).
The problem is with the forum software. It uses the wrong disposition (an HTTP protocol thing) for the stream. After pasting the image URL then appending ";image" overcomes the problem. The forum software then uses the correct disposition.
In other words, this...
...is supposed to be treated as "download this file to your computer". Firefox and Chrome do not do that. The image is incorrectly embedded instead.
...is supposed to be treated as "display this image in your browser". All three browsers work as expected; the image is display in the browser.
If you click the two links above you will see the difference.