Image re-size to 50% of original size in HTML

You did not do anything wrong here, it will any other thing that is overriding the image size.

You can check this working fiddle.

And in this fiddle I have alter the image size using %, and it is working.

Also try using this code:

<img src="https://stackoverflow.com/questions/11117602/image.jpg" style="width: 50%; height: 50%"/>​

Here is the example fiddle.