Hello,
A picture content control can include an alt text content so it can be used with replacePlaceholderImage to replace the image. Sorry, but the current stable version doesn't include a method to replace a picture content control by the title or tag value of a content control.
The modifyInputFields method only replaces input contents such as rich text content controls; so a new method should be generated for picture content controls. We have added a new task to add it in a future release of phpdocx.
Regards.