Google Image Search using Android
Instructions performed on Firefox for Android on stock Android 6.0.1.


Switch to Desktop mode in your preferred browser (it changes the user-agent). 
Open https://images.google.com and tap the camera icon next to the search bar on the page.
Tap Upload an image
From the bottom sheet pick Documents. (This option might not be available in some Android versions.)

(Click image to enlarge)


In the left pane, select Images or Internal Storage and select your image file. That's it.