Blocking Google images
Google image search is highly integrated with other search functions. So it is not possible to block just image search without blocking Google search all together. However, Safesearch applies to both regular search and image search. So the image search results are filtered by Google Safesearch so you should not see any porn images in the search results. If you still find some images inappropriate, unfortunately we cannot change how strict Google safesearch does the filtering.
Another option is use special search engines such as duckduckgo.com. But the search results might not be as good as Google.