When you are writing a post, there is an Edit Attachments button in the Post Options section below where you enter the text of your message. This allows you to create attachments quite easily, and is all most users ever need. If you want to make the image appear in-line with the post, it's a bit harder: You have to get the address of the image you attach (right click on the uploaded image) and enter that address between [ img ] and [ /img ] tags (without the spaces).