I see what's happened; you're looking at the tutorial for using images in an
article, but you're actually using a forum thread.
Articles are different from forum threads and posts, and use different coding and formatting.
For posting images into a thread, navigate to your uploaded image in the gallery (I'm going to use
this one as an example) and look for the 'Share this image' heading beneath the image itself.
For use on the forums, you need to click the text in the box marked 'Forums:' (if it asks for permission to upload the link to the clipboard, then okay it) and it should automatically copy it (if it doesn't, you can highlight it and manually copy). Next thing to do is simply paste the contents wherever you want the image to appear, such as:
Under this text:
What you should have typed for that to appear is:
Under this text:
You can also use images hosted elsewhere, and oftentimes popular image-hosting sites, such as photobucket, provide the link already wrapped in img tags, so all you have to do is copy and paste what they give you.