Edit your thread, and then scroll down to manage attachments. Click on that button and it'll open a new window. Down the bottom of that window is the previously attached images. Simply hover the mouse over these thumbnails, and you'll see a "X" come up to remove the old ones. If you don't see any of this then you must toggle advanced uploaded on. Then in the top right is a window to add new images. Once uploaded they'll appear down the bottom where the previously attached image was. Simply make sure it's selected and then "insert inline" to add to the post and close that window. That should do the trick. Thank you.