With appwarp you can only send text or binary message with a size limit of 1024 characters.
To share image files you can use App42 upload service along with appwarp. Upload the file to App42 cloud and then share the link with other user.
You can create your own JSON format and send it. It could be something like this
{
"type": "image",
"url" : "http://..."
}
or
{
"type" : "string",
"string" : "mahesh is Winner"
}
Let me know if this is helpful.