Google's Image Search received a major update with the introduction of "visual search" i.e. the possibility of searching via an image, either from your computer or from the web.
This feature though opened up all sorts of new possibilities. One very interesting use of the visual search feature as well as the modern web in general is the ability to search for an image, from a list of image results, simply by dragging it over the search box area, as Google Operating System
noted.
It's not the only way to do a search like this, but it's definitely the faster. A less glamorous way is to click on the image result, to go to the interstitial page, and then click on the "Search by Image" link there.