The problem is that you didn't load your packages. When you type
pkg list you can find which ones are loaded by an asterisk in front of their names. Load a package with
pkg load image.
Having to load a packages is that thing that most users find strange but if you compare with other languages, such as Python, Perl, or C++, would you expect them to
#include every libraries available in the system by default? See
Octave's FAQ for more details.