SOURCE

var shot64 = "data:image/jpeg;base64,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";
var shotImg = document.getElementById("screenshot");
shotImg.src = shot64;

var hiddenImg = new Image();
hiddenImg.src = shot64;
hiddenImg.onload = function() {
    alert("宽：" + hiddenImg.width + " 高：" + hiddenImg.height);
};

<img id="screenshot" class="shotbox" width="300" height="300">

.shotbox {
    border:2px solid #ff0000;
}