The easiest way is to have your images uploaded to a site such as Flickr or Photobucket, or even your own website.
I use Flickr.
Open the image you want to use on here, right click it, and copy image address.
Then within A4A, click the "insert image" icon in the list of options above the text box (second row down, far left), this will give you [img] and [ /img ] paste your image address between these two [img] symbols.
Job done.