How to block Google Images (note: Google changes their URLs often so the following is subject to change)
To completely block Google images, you will need to block two URLs: images.google.com & www.google.com/imghp by going to Website Filtering > Website Access > Add A Site.
Then, you will need to add three URL filtering rules by going to Content Filtering > URL Filtering > Add Custom Rule > Select: Exactly Matches > Type in: tbs=isch > Click on Save Rule. Click on Add Custom Rule (again) > Select Exactly Matches > Type in: tbs=img:1 > Click on Save Rule. Click on Add Custom Rule (again) > Select Exactly Matches > Type in: tbm=isch > Click on Save Rule.
Commit and Restart your changes.