I do this by defining the image in a defines file: includes/languages/english/extra_definitions/YOUR_TEMPLATE/my_image_defines.php

Code:
define('MAIN_IMAGE_WIDTH', '200');
define('MAIN_IMAGE_HEIGHT', '200');
define('MAIN_IMAGE', 'your_image.jpg');
define('MAIN_IMAGE_TEXT', 'image text');
Then calling it in includes/languages/english/html_includes/YOUR_TEMPLATE/define_main_page.php

Code:
<?php echo zen_image($template->get_template_dir(MAIN_IMAGE, DIR_WS_TEMPLATE, $current_page_base,'images'). '/' . MAIN_IMAGE, MAIN_IMAGE_TEXT); ?>
Place the image in includes/templates/YOUR_TEMPLATE/images/your_image.jpg