At this time we don't support using custom user provided images. It is something we are looking into providing in the future though. We use UserVoice to track feature requests and help gauge user demand. You can vote for this open request there to show your support and subscribe to get updates on any new developments.

One option on Ubuntu is to set up an encrypted home directory for your user. This can be done easily, even after the fact:

sudo apt-get install ecryptfs-utils cryptsetup
sudo ecryptfs-migrate-home -u user