When you created the droplet did you click on the "Add optional SSH Keys"? While it shows on the "Create Droplet" page it is not automatically added (It's gray), unless you click it your droplet will not have the SSH key.
If you didn't and still haven't logged in or modified the image I would recommend destroying the droplet and create a new on, or login with the root password should have been emailed to you. If you did not receive a email you can reset the root password from the DO portal (Droplets->Access->Reset Root Password). From there you should be able to ssh into the system and manually add the SSH key to /root/.ssh/authorized_keys (you may need to create the file file if it missing).