Is your Group Uploads folder in the media browser full of items you don't need any more? If so, you can delete uploaded media one item at a time, or in a group. To get started, select Browse Media from the Media menu at the top of the main app window and select the Owned tab on the left.
Right click on the items you'd like to delete and select Remove from Library, or select the same option at the bottom right of the details pane. To delete multiple items, hold down the Ctrl key and select up to fifty items you'd like to delete. Right click or select Remove from Library in the details pane.