Can I Use An Image For the Sidebox Header?

The short answer is yes. You can use two methods to achieve the result.

Using an image editor, create your image sets with a name for each sidebox you want to change and save your graphics to includes/templates/CUSTOM/images/ and upload it to your server.

In your includes/languages/CUSTOM/english.php find this line of code:

define('BOX_HEADING_CATEGORIES', 'Categories');

You can change it as follows:

define('BOX_HEADING_CATEGORIES', 'your_graphic_image.???');

Now in includes/modules/sideboxes/CUSTOM/categories.php Find this:

$title = BOX_HEADING_CATEGORIES;

and change it to:

$title = zen_image($template->get_template_dir(BOX_HEADING_CATEGORIES, DIR_WS_TEMPLATE, $current_page_base,'images'). '/' . BOX_HEADING_CATEGORIES, HEADER_ALT_TEXT);

Note: You would do this for each of your sideboxes.

Make the needed changes, save the files and upload to them to your server.

If you want to replace the default Zen Cart background image to be used for all the sideboxes use an image editor to create your image and save it to includes/templates/CUSTOM/images/ and upload it to your server.

Now open you stylesheet.css and find following:
.leftBoxHeading {
margin: 0em;
background: url(../images/your_background_image.jpg) #ff6699;
padding: 0.5em 0.2em;
}
.rightBoxHeading {
margin: 0em;
background: url(../images/your_background_image.jpg) #ff6699;
padding: 0.2em 0em;
}
Change the highlighted image file name in each of the URL functions , save the stylesheet and upload both files to them to your server.