When you create a poste there is a row at the bottom that says attach file. Click on browse and then select the file you want to post. Be warned the file doesn't show up if you select Preview Reply, but does show when you click Submit Reply.
Yes, the IMG at top is for a web site, but at the bottom is the "ATTACH FILE" section and you click on "Browse...". You then pick the file you want and click on "OPEN". Give it a try...