Go to a group you belong to. Sign in and go to the 'Whats New' Page. To get there, look at the very top on the left hand side in the navagation bar. Once you see it, click on it.
Once you are at the 'What's New' page look to the right side and find the box that says "Member Tools". It looks like this...
Click on 'My Storage Info' at the bottom of that box.
Now you will be at the 'File Deletion Page' and it'll look like this...
1. Click on the "My Attachment" tab at the top.
2. Next, use that drop down arrow to select a group you belong to. (You should see all the groups you belong to here once you click on that drop down arrow).
3. At the bottom it will show you all the the items you have added to the message boards and the date that you did. Put a check in the boxes to the attachments you no longer need.
4. Now just click on the 'Delete Selected Attachments' button.
Be sure to do the above steps for all the groups you belong to. It should be done every so often so that you can regain some of your storage back.
You can also check under the Pictures and Documents for files you no longer need.