I would suggest having the pink bar (that indicates the current length) grey out after 5 seconds of time, so if the selected portion is 10 seconds long have 50% of it grey.
I've used another site for this before (called Gifsoup: http://gifsoup.com/) and they charge about $5/m for the option to make longer gifs with no watermark, I think this would be something you could offer. I would definitely convert as I prefer your interface.
edit: The gif quality is pretty terrible, can that be improved?
edit edit: If you click "post to forgif.me" and then have to login and then login it loses the URL, you have to go back to find it again and click the button again.