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.