Well, the forums running here on MarbleBlast.com are powered by the Kunena plugin. As such, any of the elements you see in the 'Boardcode' section of your Create Post interface will use BBCode elements - so the IMG tag should behave as a proper IMG BBCode tag: [img.]URLTOIMAGEHERE[/img]
It's possible you're not pulling the proper BBCode url embed from imageshack or imgur, and as such the BBCode cannot interpret what the url of the image location is. If it's simply showing the link, then the forums cannot find an image file at that URL location and will instead display the URL that you provided inside of the [img.] tags. Other forums handle this differently (the old Proboards forums would simply show a blank box with a broken image link icon).
When inserting an image into your post from a third party site, be sure you include the DIRECT LINK for that image, and not a reference to the image. I know for a fact that some sites such as Photobucket, Imageshack or Imgur will give you their REFERENCE URL by default, forcing users to visit these third party sites in order to view the image. However, you want the DIRECT URL to the image element itself if you wish to embed it properly in your post.
Hope this clarifies things, @CheepFishMBP please let me know if you're still having troubles getting it to work, or provide the URL you're attempting to use so I may further troubleshoot the issue.
~Aayrl