A local source is your hard drive. A remote source is something like Facebook or Flickr etc.
Presumably someone has helped the OP to understand how to post photos on the forum, aside from telling him that he must host them somewhere else online.
If not, here's how you do it:
Place your photo somewhere else online.
Determine the URL of your photo where it is online (carefully avoiding using the URL of the page your photo is residing on).
Copy the URL of your photo to your clipboard.
Return to the forum and click on the pale green square icon in the bar at the top of the message window where you wish to put the photo.
Choose remote source and paste the URL into the available window.
Unclick the little box below the URL line.
Click on OK or whatever it is and the photo should appear inside the message box.
For more guidance, ask again.
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