I am looking at the zenpage theme and would like to know how to make a few modifications.

1. How can I change the font of the gallery title text? OR how can I replace it with my own logo? (I have tried swapping 'header.jpg' in /themes/zenpage/images/ with my own 'header.jpg' containing my logo, but that had no effect).

2. On the 'Archive View' page the 'Gallery Archive' is displayed as 1 long column. Is it possible to change this to 2 or 3 columns to reduce the length of the display?

OR

Is it possible to keep the display as 1 column and have the 'Popular Tags' displayed in a second column next to it?

I appreciate that anything is possible for people like you with the required knowledge, but what is possible for unskilled novices like myself?

1) That should work. Did you refresh the page? (sometimes you need to refresh more than once)
Note that you'll need some CSS modifications too otherwise it will look ugly.

2) and in general:
Anything is possible (almost) but requires some basic knowledge of HTML, CSS and PHP.
Zenphoto is meant to be used as a starting point and provides a few themes as examples.

Unfortunately we simply lack the resources to provide more themes and there are also just a few 3rd party themes that are actively maintained.

If, after this, you still want to give it a shot, this is the place to start:
http://www.zenphoto.org/news/theming-tutorial/