Deleting Google Image Search History
There are two cases:


You are logged in your google account on your device browser
You are not logged in


In case 1, go to www.google.com, scroll down and click on "settings" at the bottom. You'll get to a page which gives you the option of wiping your search history

In case 2, go to www.google.com, start typing something in the search box, you'll see in the autocomplete the option to wipe your search results.

What Matthew suggested wipes the search history of your device not on google's servers.

You have to do both if you wanna clear everything.