Follow the link.  It explains the different ways you can use it to search the web for other uses of pictures.  I recommend using the extension that installs in your browser as it is so easy to use.  It is not foolproof but it is probably the easiest way to find out if an ad is using pics that show up elsewhere.

Thanks for showing people how to be more effective in their screening.  This is the type of advice newer members of our community should always pay attention to.