There is no reliable way of doing this. Whatever automated process you use will generate false positive and false negatives. If you're willing to accept that you could look at the Image Nudity Filter, which analyzes images based on skintone. It will tend to flag images that are close-ups of faces too however.
Such a method might be effective in conjunction with information about a user's history. If they suddenly upload hundreds of images (when they might average two a week prior to that) and most of them get flagged then it's probably want to suspend those uploads until someone looks at them.
Come to think of it, simply uploading a large number of images in a short period of time (and bringing that to the attention of the site admins) is probably sufficient most of the time.shark deluxe steam mop