When it comes to registering screenshots with Mythic, the process is almost identical to File Downloads (Agent -> Mythic); however, we set the is_screenshot flag to true in the download portion of the message: 
{"action": "post_response", "responses": [
    {
        "task_id": "UUID here",
        "download": {
            "total_chunks": 4,
            "host": "hostname the file is downloaded from",
            "is_screenshot": true //indicate if this is a file or screenshot
        }
    }
]}