It's simple: if you choose to answer my post, below the box in which you enter your text you have a label "Attachments:" and a link "Attach a file". You choose the latter (click on it) and a little window appears with a button "Browse" or so (for me it is actually in Polish!). When you click on that button a new window pops-up which allow you to choose the file (scan of the document) from your PC hard disc. You choose the file and press the button "Open" and that's it!