Hello,
The method removeTemplateVariable only works for text variables because it looks for w:t tags, and image placeholders don't use this tag.
The DOCXPath suggested solution works fine. We recommend you to open a support ticket (http://www.phpdocx.com/support) and one of the members of the dev team will create a custom script for your template. In the support ticket, please attach the template you are using and they'll create the script using it.
If you don't want to get support tickets, you can send the template to contact[at]phpdocx.com but the script will take longer.
Regards.