Whatever user you created at the time of creating the image, the Ubuntu installer apparently ignored it, because it didn't create a
/home directory for that user. So apparently you have just one regular user on that system, and that is the user you created when you were installing the customized Ubuntu OS and you also should be able to login without a password as a guest user.
While installing the customized OS, the Ubuntu installer asked you to type your user name so that it could create a
/home directory for a new user with that user name. Then it asked you to type a password for that user or else if you left the password field blank you can login without a password just by pressing the Enter key.
If for some reason, the Ubuntu login screen does not accept your password for the new user that you created in the Ubuntu installer, then you can change the password for that user from an Ubuntu live DVD or live USB. The login password is normally the same (by default) as the administrative password which is used to run
sudo commands in Ubuntu. The instructions for resetting the administrative password are found in the answers to this question: How do I reset a lost administrative password?.