Funny - I'm about to compose a hub on this very thing, since I've now learned an enormous amount about licenses, creative commons images, and copyrights.
It's probably wisest NOT to use Google Images. The largest percentage of images on Google are all copyrighted. This is also true of most of Flickr.
I mostly use one of two sites:
1) search dot creativecommons dot org - type your keyword(s) into the box, then click the button of the resource you want to use. But be very careful of Flickr. Even if some of their images come up as a result of this search, some of them still require you to purchase a license. You can tell by the icons on the middle of the right column.
2) wikimedia commons dot org - pretty much everything on this site is in the public domain, and therefore completely free to use.
Make sure you attribute them appropriately on your hub, by the way. The Learning Center (click the help button) can help you with that. I usually fill in all the fields in my photo capsule these days, unless they're my own images.
There are others - just google "free images" or "free pictures" then be sure you read their guidelines carefully. Good luck!