As promised, Google Images added the option to restrict the results by aspect ratio. If you go to the advanced search page, you'll be able to choose one of the four types of images: tall, square, wide and panoramic.
This is one of the last missing features from Google Images Search that were available in Microsoft's Bing. Google Images has improved a lot lately by adding advanced options that help you refine the results (Creative Commons search, searching by size) and options that require image analysis (finding similar images, searching by color, detecting faces).
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