OpenDNS cannot filter specific images from search engine results (except for Bing). However, OpenDNS can filter all image results on a network from appearing if the network Always Block list has been configured correctly. The one exception to this is Google Images, which is explained in more detail further down.
The following is a list of domains and subdomains that popular search engines use to host image search result content. Blocking these domains and subdomains will block image search results on their respective image search engine.
Google: Google embeds images as data in the HTML source, which cannot be blocked by OpenDNS unless you block google.com as a whole. As an alternative you might try using Google SafeSearch. Google SafeSearch may be able to be enforced with the instructions on the this SafeSearch article.
Most image may be blocked by blocking the following:
Additionally, Bing stores their explicit images on a single domain: explicit.bing.net. Therefore, you can add this domain to your network's blacklist and filter out explicit images from a Bing search.
Please note, blocking images of a search engine may cause other search engine functionality to not be displayed correctly.