Currently, unless it was changed recently without my noticing, there is no way to change it without just uploading a new image. The featured submission is always just your most recently submitted image. I emailed them suggesting that they allow it to be changed, and they said that many others have done the same and they still haven't yet decided weather or not to allow users to change it.

It's planned to be added. At least it's in the todo list from the forum.