Hello,
Yes, you can set image properties when using replaceVariableByWordFragment. You need to set float and imageAlign properties as center and use a not inline textWrap with addImage. Also please set target as block with replaceVariableByWordFragment to overwrite the default align of the paragraph.
About replacePlaceholderByImage, we have done some tests and everything is working correctly. Please run the example included in the package (examples/Templates/replacePlaceholderByImage) and check it. If you still are unable to use the method, please send to contact[at]phpdocx.com your template and we'll check if the placeholder has been added correctly.
Regards.