When you write a post, you're using BBCode, which is why you only see text.
To post an image with the IMG tags you need an image URL such as http://www.seibertron.com/images/factio ... eckers.png
What you want to do is include that URL between [ img ] and [ / img ] (with no spaces) to obtain this:
The faster ways are to either click the IMG tag button and paste the URL between the two, or paste the URL, highlight it, and then click on the IMG button.
To obtain an image URL, use one of the online free image host websites, like Photobucket, Imgur or Imageshack (or even Flickr, but make sure privacy settings allow everyone to view the image, or it won't work).
Alternatively, you can also attach the images by selecting the Upload Attachment tab under the Preview and Submit buttons on the posting editor.