External resources such as images or PDFs need to be processed separately from the JSON or XML request body. In order to include an image on a form (either during creation or updating after the fact), the following actions need to be performed.

Upload the image using the Files endpoint. See Uploading a file for more details.
After the image has being uploaded, obtain the Id of the file.
Include the Id as the value of the id property on the image field when creating a new form or updating an existing form.