Установить фон на canvas

Можно так:

<canvas id='canvas' width=250 height=250></canvas>
<img src='http://900igr.net/up/datai/165870/0003-002-.png' id='img'/>

JS:

function ge(id) {
  return document.getElementById(id);
} 
const canvas = ge('canvas');
const ctx = canvas.getContext('2d');
const canvasWidth = canvas.width;
const canvasHeight = canvas.height;
const img = ge('img');
for (let i = 0; i < canvasWidth; i += 50) {
  for (let j = 0; j < canvasHeight; j += 50) {
    ctx.drawImage(img, i, j);
  }
}

Пример на JSFiddle