You can add in your image inside the div like ive done below. Just change the image name and alternate text to your own images and it will show up. As long as your image is in the default "images" folder, it will work.
<div class="contentContainer">
<div class="contentText">
<?php echo tep_image(DIR_WS_IMAGES . 'yourimagename.jpg', 'Alt Text For Image');?>
<?php echo TEXT_MAIN; ?>
</div>
So above, you will see your image before the main text.
If you want it after it, just move your image to after the <?php echo TEXT_MAIN; ?> tag.