The short answer is that it is possible that the Google images are infringing copies of stills and if they are then your use would be further infringement.
If the images have been lawfully obtained for the purposes of posting on the Google images website then the further use by you may not be approved by the original permission obtained by the poster to the Google website.
So you have to find out under what terms the images have been posted to Google images. These terms are likely to be available on the Google images website. You are restricted by these terms and there would appear to be no fair dealing or education exception which would assist you.
You should refer to our sister service JISC Digital Media - http://www.jiscdigitalmedia.ac.uk/. as they may be able to find you images which you can lawfully use.