Attaching an image requires you to click the "Manage Attachments" button while posting a new thread or reply to a thread. This button is below the text box where you type in your post.
To post an image in a post without uploading it you need that image's address in the format of "http://www.example.com/folder/image.jpg". Once you have that image you need only click the small yellow button that looks like a mountain with a sun above it below the drop-down box that says "[Size]" in it above the text box you type your post in. Get that image address put in the box that appears (Either by typing it out or copying and pasting) and then the proper code will be made for you.
