There are a couple of ways to do it. The easiest is to double click on the picture before you submit the post. A box will pop up. Click on the size you want (biggest one). then submit post / reply.
Other option is to click on the attached picture to enlarge it. Then go to the address bar and copy the URL for the pic. Then when you are posting, click the picture frame in the row below and to the left of the attachment paperclip. Paste the URL of your picture, uncheck the box, then click "OK".