I'm having a hard time figuring out what you mean by "switch the search button ... to an image instead of a button." Buttons can be images or text. If it's currently an image (default ZC set up), just replace it with another that you like better, in the appropriate images folder.
But if you're using css buttons, and the search button is therefore just text, then the answer is almost the same. :smartalec:
To override a single CSS button by an image button:
Copy the image button to:
includes/templates/YOUR_TEMPLATE/buttons/english/
For example copy:
includes/templates/template_default/buttons/english/button_search.gif
to:
includes/templates/YOUR_TEMPLATE/buttons/english/button_search.gif
(You can, of course, edit this image so it looks like whatever you wish. But user studies suggest strongly that it should say "Search" or whatever is appropriate for the language your site uses, no matter what it looks like.)
Be sure to turn on Zen Cart's css buttons under admin / Configuration / Layout Settings / CSS Buttons. Otherwise your search button can not be changed to a css button.
HTH
Rob