A Google Webmaster Help thread has one webmaster who is upset that many, about 4,000, of his images are being blocked by Google's Image Search SafeSearch filter.
Gary Illyes from Google explained that although the images might not look visually offensive but the filenames and words on the page can also flag the image as offensive. He said, do a site command and add the word [naked] to the query and that shows that many of those images are returned for a type of nude keyword phrase.
The easy fix is to rename the filenames of the images to safe words and to remove any unsafe words from the context of the page.
Gary explained in the thread:
The SafeSearch algorithms look at a variety of signals to decide what's potentially unsafe, and one of them is the filenames of the images. If the filenames contain words that may be offensive, generally the image is also potentially offensive. The same applies to the textual content of your HTML pages.
I realize identifying the images and/or the pages that may be filtered can be a challenging task; what I usually do is to turn off SafeSearch and do searches for [site:YOURSITE.com naked] and similar keywords to find them. The results that show up are typically related to the searched keyword.
To fix the issue, you will want to remove the potentially offensive words from the image filenames and the text of the landing pages.
Forum discussion at Google Webmaster Help.