For bugs relating to the image itself,
File ubuntu-cdimage bug
If there is an issue with an application itself, please report the bug directly against that application. You can use apport to gather logs to attach to your bug report.
First install apport to be able to collect logs from your chroot.
apt-get install apport
Then execute ubuntu-bug against the package in question
ubuntu-bug (application)
If this fails, move on and file the bug without logs, taking care to describe the issue.
