Problem load images REST API

Salutations,
I'm creating my document starting from a template using the API https://api.eu1.echosign.com/api/rest/v6/agreements The whole operation is successful ... all fields are populated except the fields of type image. I am trying everything, for example

"mergeFieldInfo": [

.....

{
"fieldName": "xxx",
"name": "xxx.png",
"mimeType": "image / png",
"url": "https://xxx.xxx.xxx/xxx/xxx/xxx.png"

}

or

{
"fieldName": "xxx",
"name": "xxx.png",
"mimeType": "image / png",
"defaultValue": "data: image / png; base64, iVBORw0KGgoAA ....."

}

or

{

"fieldName": "xxx",
"name": "xxx.png",
"mimeType": "image / png",
"default": "data: image / png; base64, iVBORw0KGgoAA ....."

}

How do I resolve?