Hello,
Our apologizes, image's links are using their own tags instead of the used by texts so it can't be replaced easily using the current version of phpdocx.
You can set a placeholder such as $VAR_IMAGE_URL$ in the URL image field and this var may be replaced by a new value. Please send us an email to contact[at]phpdocx.com and we'll send you a patch to be able to do this kind of replacement.
Regards.