There are several ways to do this. One way is as follows:
First, put your previous and next buttons in your '/images' folder under your Zen Cart installation.
E.g. '/images/prev.gif' and '/images/next.gif'.
Next, override the file /includes/languages/english.php as follows.
Replace the following two lines:
PHP Code:
define('PREVNEXT_BUTTON_PREV', '[<< Prev]');
define('PREVNEXT_BUTTON_NEXT', '[Next >>]');
with these two lines. Adjust according to your actual setup (e.g. actual image file name).
PHP Code:
define('PREVNEXT_BUTTON_PREV', '<img src="images/prev.gif" alt="<< Prev" />');
define('PREVNEXT_BUTTON_NEXT', '<img src="images/next.gif" alt="<< Next" />');
Finally, refresh your browser and verify the changes.
Do note that I've only included the basic HTML img attributes, you should add any other img attributes (e.g. title, width, height) according to your own needs and other best practices.