Apple - How to search by image from the iPad?
When you're on the Google Images page scroll all the way to the bottom. There is a choice there to use either Google Tablet or Classic (defaults to tablet on the iPad). Click on classic and you should get the same options as on your mac/pc.
If you don't mind installing and using Google Chrome browser, you can tap-hold on any image and the "search google for this image" link is right there. Imho the easiest option.
Here is an app-free way to perform Google Image search on Safari (ios devices)
Tap and hold on the picture you want to search and choose Copy to take its link.
Go to google.com, choose tab Image and paste the link into the search box and enter.
You will see the text For matching images, try search by image right under the menu bar . Tap on "search by image" and here we go!
Hope this help!