This API version isn't the latest one.
It is advisable to use the latest API version which contains the latest Context.IO enhancements.
All API calls returning data about messages include a personInfo property which list additional information about the sender and recipients of the message.
{
emailAddress_1 : {
thumbnail: (string) thumbnail URL
},
emailAddress_2 : {
thumbnail: (string) thumbnail URL
},
...
}