Hello,
The only case where phpdocx could return that exception when adding an image is if the image doesn't have a dpi or it's set as 0, but it should return the following exception: There was an error adding the image. In this case you can set a custom dpi using the dpi option.
If you send to contact[at]phpdocx.com an image that illustrates your issue, we'll check it.
Regards.