Canvas.htm

<script type="text/javascript">
window.onload = function() {

    var oCanvas = document.getElementById("thecanvas");
      var oCtx = oCanvas.getContext("2d");

    var iWidth = oCanvas.width;
    var iHeight = oCanvas.height;

    oCtx.fillStyle = "transparent";
    oCtx.fillRect(0,0,iWidth,iHeight);

    oCtx.beginPath();
    oCtx.strokeStyle = "rgb(255,0,0)";

    oCanvas.onmousedown = function(e) {
        bMouseIsDown = true;
        iLastX = e.clientX - oCanvas.offsetLeft + (window.pageXOffset||document.body.scrollLeft||document.documentElement.scrollLeft);
        iLastY = e.clientY - oCanvas.offsetTop + (window.pageYOffset||document.body.scrollTop||document.documentElement.scrollTop);
    }
    oCanvas.onmouseup = function() {
        bMouseIsDown = false;
        iLastX = -1;
        iLastY = -1;
    }
    oCanvas.onmousemove = function(e) {
        if (bMouseIsDown) {
            var iX = e.clientX - oCanvas.offsetLeft + (window.pageXOffset||document.body.scrollLeft||document.documentElement.scrollLeft);
            var iY = e.clientY - oCanvas.offsetTop + (window.pageYOffset||document.body.scrollTop||document.documentElement.scrollTop);
            oCtx.moveTo(iLastX, iLastY);
            oCtx.lineTo(iX, iY);
            oCtx.stroke();
            iLastX = iX;
            iLastY = iY;
        }
    }
}
document.write("<canvas width=\"356\" height=\"700\" style=\"border: 1px solid black; background-image:url('http:\/\/i.imgur.com\/fI3qz.jpg'); position:fixed; top:10px; left: 10px;\" id=\"thecanvas\"><\/canvas>");
</script>