There are two ways you can go about it.
One, you can do the 'add attachment' method, which is at the bottom when you make or reply to a post. Pretty basic file loading, just click 'Browse' and find the file on your computer.
The other way is by using the IMG tags, in that you would take the URL of an image, such as this one-
But place [ img ] and [ /img ] around it (remove the spaces), getting you this --
Either way works the same, however doing it the 'Add Attachment' way adds the little green camera icon next to your post.