The best way would to visit images.google.com then click on the photo camera icon to get the following screen as in the screenshot:
You can either use an image URL or an image file to find its duplicates in same size or different sizes. The results will look like google search results with a list of websites where you can find the image that you either uploaded or the image URL that you entered. You will be able to see all the websites on which this image file is visible.
