Current implementation allows us to use MergeField and replace that (text) field with image.
By supporting images (with specific title or description) as placeholders, users would be able to format that placeholder (rotate it, add border, shadow, etc) and MailMerge would replace that image with actual content.
I'm aware that we can add MergeFIeld before of after image and manipulate word from C#, but this is just a temporary solution. I think image placeholder would be elegant and more intuitive way of doing this.
This feature request has been implemented and is available in the latest versions of GemBox.Document.