It seems like the method for reporting offensive images in Google Image search has been dropped out of the new interface that launched in late January.
Typically, if you spot an offensive image within your image search results, you would have been able to click on "report this image" or something like that within the image search results.
That functionality is no longer available in the new interface.
Now, it seems you have to go to an external form to report offensive images found in Google Images SafeSearch.
This is how it worked last year:
Now, that option is no longer there. I am surprised Google doesn't simply put a button within the preview screen, there is plenty of room next to the "image details" button:
This was reported in the Google Web Search Help forums where Googler Kousha said:
Thanks for bringing this up. We're aware that the "Report Offensive Image" option isn't appearing where it used to; we're working on bringing it back. In terms of the help content, I'll look into updating it to make it as accurate as possible.
Forum discussion at Google Web Search Help.