As the title suggests I want to change the top image of the cube to my own image (my company loge).

How do I go about this?

There are two possible ways:
either replace cubecap.png in /usr/share/kde4/apps/kwin/ or modify kwinrc and change (or add) option "CapPath" in the cube effect settings. As you are using Kubuntu I'd recommend to have a look at the kubuntu-default-settings as they are changing the image, too.

Btw. there is no gui option to change the cap image, so that the cap can be used for branding.

Thank you for that. I knew is was something like that but wasn't sure which file to edit