Ubuntu and Debian require libimlib2 as dependency.
Please don’t forget to refresh your sources by apt-get update.
Also ensure that all your repos are active. If you don’t have the default shipped repos this might fail.
Also note: libimlib2 is not provided by the Zammad-Repository.
This is an OS based issue that we cannot handle, I am sorry.
We ensured that it is working on CentOS, Debian, Ubuntu and openSuSe and I verified that again yesterday.
It has to be about your repositories.