Because that heading is also a link, it will still have the [more] after/below the image. You can redefine the text to blank, but then all other sidebox headings that have the [more] will lose it too.

Instead of manually editing PHP files for each sidebox, the easiest way is to install the Image Titles mod from Free Addons (link in my sig), and upload the image to /includes/templates/your_template/buttons/english/ with the filename boxhead-shoppingcart.gif.