ModerateContent.com

<ul>
<li>blocks any image uploaded with inappropriate content (nudity etc)</li>
<li>leverages the built in WordPress upload mechanism</li>
<li>works with most other upload plugins</li>
<li>leverages the FREE api at <a href="https://moderatecontent.com" title="The FREE image moderation API" rel="nofollow ugc">ModerateContent.com</a></li>
<li>bbPress support</li>
<li>external image url review</li>
</ul>


Image (NSFW)

Stops the upload of NSFW images. Using the FREE api at moderatecontent.com to rate content and block it if it's inappropriate.

Freika

<p>This plugin allows to change opacity of all  elements on page to 0.1. This may be useful if your site contain NSFW(Not Safe For Work) images and you do not want make your visitors close this site. Just place NSFW button anywhere you wish or use build-in button(left-bottom).</p>


NSFW WP Images

NSFW WP Images plugin. Allows (almost) hide images on any page.

Image (NSFW) vs NSFW WP Images