Hello,
The replacePlaceholderImage method replaces an existing image that includes a template placeholder in the alt text with a new image. On the API doc page (https://www.phpdocx.com/api-documentation/templates/replace-placeholder-image-Word-document) you can read this same information.
If you want to replace a text placeholder by an image (or a chart, a table, a list, complex contents or others) you need to use replaceVariableByWordFragment. We recommend you check the included samples of this method and also the following documentation pages:
- Practical guide Pictures and images (Replacing images in a template section): https://www.phpdocx.com/documentation/practical/pictures-and-images
- Practical guide WordFragments and WordML: https://www.phpdocx.com/documentation/practical/wordfragments-and-wordml
Regards.