They don't need to be pure real or imaginary. Any "mixed" complex number works so long as [tex]z=z'[/tex].
Let [tex]z=z'=a+bi[/tex]. Then
[tex]|z+z'| = |2a+2bi| = 2 \sqrt{a^2+b^2}[/tex]
[tex]|z| + |z'| = 2|a+bi| = 2 \sqrt{a^2+b^2}[/tex]
so [tex]|z+z'|=|z|+|z'|[/tex].
The geometric interpretation is essentially identical. [tex]|z+z'|=2|z|[/tex] is a complex number twice the distance away from the origin in the complex plane as [tex]z[/tex], which is exactly [tex]|z|+|z|[/tex].
