if you want to add a png image, you have to get the latest jspdf.js and add the support png libraries
<script type="text/javascript" src="libs/png_support/zlib.js"></script>
<script type="text/javascript" src="libs/png_support/png.js"></script>
<script type="text/javascript" src="jspdf.plugin.addimage.js"></script>
<script type="text/javascript" src="jspdf.plugin.png_support.js"></script>
<script type="text/javascript" src="jspdf.js"></script>
then in the script, change the format to 'PNG'
<script>
var doc = new jsPDF();
var imgData = 'data:image/png;base64,iVBORw0KGgoAAAANSUhEUgAAAMgAAABy...
doc.setFontSize(40);
doc.text(30, 20, 'Hello world!');
doc.addImage(imgData, 'PNG', 15, 40, 200, 114);
doc.output('datauri');
</script>
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