Google Image Search has a new feature that shows a list of 7 related images when you select a result. The section is called "try these too" and is a clever way to integrate the similar images feature. Click one of the related images and it will replace the original result. The first image from the list is always the original search result, so you can always go back to that image.
If you want to find a more comprehensive list of similar images, click "more sizes", then click the "More sizes" dropdown and select "Visually similar".