TeraByte Unlimited

As I understand, one should add the disk group to their user account to run ./imagel as a plain old user, which I've done.

I fire up the app, tell it I want to do a backup, select full, and the drive interfaces available are: Linux, and Virtual drive.

I tried running it as sudo, and I get an error telling me it can't find the display.

you have to run via sudo. the message is a common Linux message, search internet for example on how to fix.
If you're not using the GUI version you may want to do that too.

Did you use sudo with this...

sudo adduser yourname disk