This really depends on the template you are using, but generally, you can change the logo graphic by editing the main_page.tpl.php file in whatever template's directory you are using.

Look for :

 tep_image(DIR_WS_IMAGES . 'oscommerce.gif', 'osCommerce')

Change oscommerce.gif to your actual image file name. Change the osCommerce alt text to be whatever you want.