C# OpenXML 2.5. I want to replace a merge field with an image

Question

I have a requirement to insert a mergefield say << MERGEFIELD Image >> and replace it with the actual image. I know how to loop through FieldCodes and when the field name matches with the merge field name, replace it with a TEXT value. But my requirement is to Give the users option to use a Mail merge field on the document, but then I need to replace the mail merge field with an image. Can anyone help on this with a working example.