First you should read http://support.google.com/images/answer/1325808/?hl=en& and read up on how to use Google Image Search in general. You basically can upload pictures to the search, or you can search a pic that is uploaded to a website by using said picture's URL. All you have to do if you have the URL is click the camera icon and type it in.
**RIGHT CLICK ON IT**
Or you can probably press the "Menu" button. What you are looking for is "Copy Image Location", kinda like this...
Now there are rules for this. First off, the bigger the picture the better. Don't think that a person's private profile thumbnail pic is big enough. Most times when the pic is small, Google Image Search can't process it. Matter of fact, GIS isn't foolproof at all. There are many ways someone can alter a pic in Photoshop to where Google cannot match it with literally the identical photo. So dont be convinced by not getting any results at all, CATFISH are slippery... and Google is not all knowing. Even tho its close lol
Also remember that this is a tool, not a means to harm people. Some folks may have had their private pics aired on "www.FUCKmyEXgf.com" or something. Doesn't mean that ain't them. Just means you have to be willing to believe that type of story. I'd ask for more pics, more proof, more facts. But knowing AirG, most of them ARE CATFISH lol...
Here are some basic screenshots of the process AFTER you have copied the image location (URL):
Click the Camera Icon
Paste the Url in the Box and Click "Search by Image"
There are other sites for this as well, just Google "Search By Image" to find them.