You literally then drag and drop the image into the search box.

I haven't tried it on my PC/notebook, but it works fine using Firefox on my Mac.

I thought the OP meant using Google images to verify the legitimacy OF providers not to providers.  At any rate, whichever he meant, I've found that there are times when good pictures are difficult to come by.  For whatever the reason, I've seen profiles go up from the get go and lack a link to a website--possibly because the website expired on BP by the time the review went up to be sure, but there are other situations where profiles aren't always as high quality as they should be or as accurate, and should be corrected but are not.

Verifying the client to the provider is another subject and the threads that are up every week here can easily found by just going back a page or two if that's even needed or seraching verification the erotic review in google.

OP asked this:

I've seen multiple mentions of using Google Images to verify the legitimacy OF providers, but could someone please explain to me how they do it?

I thought he might mean simply getting more or better pictures of the provider.

In case this helps, when using Google images I've found one of the quickest and most useful  things to do is to put the name of the provider and the phone number in
Google or Bing's search box.

Sometimes if there's a website, using tineye helps.

This has cleared things up some.