Google removed the "View Image" button on the 15th of February. This is a bookmarklet that allows you to download the full-size photo you're currently viewing.
Tested on Chromium 63 and Firefox 57.
- Head over here and drag the bookmarklet to your bookmarks bar.
- Go to Google Images, click on any image to view it, wait for the preview to finish loading, then click on the bookmarklet. The full-size photo should open on a new tab.
- Enjoy
If you'd like a browser extension or a userscript, @devunt has done just that.