Hi there & welcome to the challenge!
After you upload your card to the gallery, click on the card. Then copy the URL of your card from your browser address bar. Go back to that challenge's chatter thread, click reply, and paste the URL into the thread along with anything you'd like to say about the card.
Don't forget to use the keyword's challenge in the keyword field in the gallery and to choose the Hope You Can Cling To Challenge Gallery 2012 when you're uploading