- The IMG Tag does not have a width and height attributes.
- The image is not displayed at it's pixel for pixel native resolution.
You should fix the first one, so that the browser can render the area of the image right away, and it stops the footer of the page from snapping down when it loads the image. You can fix the second, by using some JavaScript and take the image width and height and divide by 2. If you don't want to do either of these things, then you could just set the width and height to 50% and it will scale correctly, but you'll only want to do that for retina displays so you will still need the JavaScript for that.