That seems to be limited to the Chrome browser. Doesn't work on IE or Firefox.
As to context menu -- right, that is Chrome-specific.
However, in any browser you can do this:
1. Go to google.com.
2. Search for anything... just anything at all.
3. On the result page, switch to the Images tab (second one, after All); now camera silhouette appears next to the magnifying glass icon in the right corner of the search box (step #2, above, is necessary because if you just go to Google and switch to Image search, camera silhouette/icon does not appear for whatever reason).
4. Click on it; a pop-up box appears with options to enter an image by its URL, or by uploading it.
What Chrome does through "Search for an Image" context menu item is just a shortcut for the search by URL. Its algorithm of searching for "similar images" is pretty good; yes, sometimes returns garbage, but sometimes is just uncanny... It's the same thing found in Google Goggles Android app.