Hi there !
Hmmm, strange. It used to work without rmagick. Try to install rmagick:
$ sudo gem install rmagick --version=2.13.1
By the way, I have a long waiting 1.2.0 release with lots of improvements and bug fixes (the latest being a grid.js to edit tabular data).
I will run all the tests on the latest head and release 1.2.0 this evening. This will probably be one of the last feature releases before migrating to rails 3.x.
And then I should find some time to update the videos (they are really out of touch now).
The 1.2.0 branch has been running in production for more then 6 months now, it's really time to roll it out. By the way, we are approaching 100k entries in the versions table (70k nodes) in one of our clients ERP and Zena is really fast thanks to the MySQL folks. I know a lot of people argue against this database, but it is really doing a great job for us.
Cheers,