When you click on an image .. there is a frame.. is there a way to maximize this image or at least 90% of what screen it is being viewed on.

This is just an example.. but when you click on the zenphoto gallery.. eventually you will get this window:
http://www.zenphoto.org/zp/theme/example/album.png.php

My dafult is small now.. can I make it larger.. if so where do I make this change.

THANKS!

There is a plugin that allows the viewer to select the size of the image. But if you wish to base the size on screen size you will have to modify your theme. There is a function for making "max sized" images to which you pass the size you want.

But if you really only want to change the default, that is a back-end option on most themes.