Greasemonkey script adding download button to bilibili paint board. You can find the download button at the description area. Please load the pictures first.

BilibiliPlace-Greasemonkey.js

// ==UserScript==
// @name        bilibili place download
// @namespace   BPD
// @include     http://live.bilibili.com/pages/1702/*
// @version     1
// @grant       none
// ==/UserScript==
var timeoutSeconds = 10;

function bpd() {
  var canvases = document.getElementsByTagName('canvas');
  var activityBlock = document.getElementsByClassName('activity-introduction') [0];
  var downloadDiv;
  if (document.getElementById('downImageLayer') === null) {
    downloadDiv = document.createElement('div');
    downloadDiv.id = 'downImageLayer';
    activityBlock.appendChild(downloadDiv);
    document.timingPBlock = document.createElement('p');
    document.timingSpanBlock = document.createElement('span');
    document.timingSpanBlock.id = 'downloadUpdateClocker';
    document.timingSpanBlock.textContent = timeoutSeconds;
    document.timingPBlock.textContent = 'Seconds till next update: ';
    document.timingPBlock.appendChild(document.timingSpanBlock);
    downloadDiv.appendChild(document.timingPBlock);
  } else {
    clearInterval(document.timingSpanClocker);
    downloadDiv = document.getElementById('downImageLayer');
    $(downloadDiv).empty();
    downloadDiv.appendChild(document.timingPBlock);
  }
  for (var i = 0; i < canvases.length; i++)
  {
    var canvasDataStr = canvases[i].toDataURL();
    var newLink = document.createElement('a');
    var brline = document.createElement('br');
    canvasDataStr = canvasDataStr.replace('image/png', 'image/png;headers=Content-Disposition: attachment; filename=' + i.toString() + '.png');
    newLink.style.color = 'white';
    newLink.textContent = 'Download #' + (i + 1).toString();
    newLink.href = canvasDataStr;
    newLink.target = '_blank';
    if (i != 0)
      downloadDiv.appendChild(brline);
    downloadDiv.appendChild(newLink);
  }
  document.iUpdateDownloadNotice = timeoutSeconds;
  document.timingSpanBlock.textContent = document.iUpdateDownloadNotice;
  document.timingSpanClocker = setInterval(updateNotice, 1000);
  document.iFlushTimeout = setTimeout(bpd, timeoutSeconds*1000);
}
function updateNotice() {
  document.iUpdateDownloadNotice--;
  document.timingSpanBlock.textContent = document.iUpdateDownloadNotice;
}
bpd();