Do NOT delete the old picture. Now there's no pic to go along with the previous post/thread. We have the server space.
Simply go to User CP in the top left corner of every page. Go to the bottom left column and click on "Attachments". That's a list of every pic you've ever posted. Find the one you want to use, click on it, and then copy the URL.
Click on the "Insert Image" icon. That's the one that looks like a postcard. Paste the URL into the dialog box, and you're done.