As you're not showing what you actually use as arguments for the drawImage() method the answer will be general for two scenarios:
If the image cover the complete canvas you can use something like this:
ctx.beginPath();
ctx.strokeStyle = '#f00'; // some color/style
ctx.lineWidth = 2; // thickness
ctx.strokeRect(0, 0, ctx.canvas.width, ctx.canvas.height);
after drawing the image.
If you use different size and position for the image just use the same values for the strokeRect()
method:
ctx.drawImage(img, 100, 100, 250, 100);
ctx.strokeRect(100, 100, 250, 100);
or
ctx.drawImage(img, x, y, w, h, 100, 100, 250, 100);
ctx.strokeRect(100, 100, 250, 100);