I’m not sure about how to get this question posted/answered; but I’ll fire away:
You can used PDFNet to associate an image with a form field, however
FDF itself can't be used to store the image. In this case you could
save PDF data to a custom format (e.g. XML based) to preserve extra
image data and metadata which you would later use to fill in forms.
In case you are primarily interested in filling PDF forms and need to
fill form fields with images you may want to refer to the following
or search for “How do I set image as a value of a button form field?".
This specific sample is importing another PDF page as a form field
appearance however you can use element_builder to import other image
types (TIF, JPEG, BMP, PNG, etc) as shown in AddImage sample project
For more information/examples of how to set custom appearances on
annotations/widgets you may want to search the forum using
"SetAppearance" as a keyword.