If you are in an editing session and call to the macro image place holder logic, you are often trying to generate an image for a macro that is not yet saved. If you try and pass the parameters through to a servlet to generate an image you can get in to a situation where you need to pass more information than a standard http query string will allow.

Allowing a macro ImagePlaceholder to provide a data url for the image would allow for the image place holder to render unsaved data.