You want to log requests for images on your site, except when they're requests from one of your own pages. You might want to do this to keep your logfile size down, or possibly to track down sites that are hijacking your artwork and using it to adorn their pages.
Use SetEnvIfNoCase to restrict logging to only those requests from outside of your site:
<FilesMatch \.(jpg|gif|png)$> SetEnvIfNoCase Referer "^http://www.example.com/" local_referrer=1 </FilesMatch> CustomLog logs/access_log combined env=!local_referrer
In many cases, documents on a web server include references to images also kept on the server, but the only item of real interest for log analysis is the referencing page itself. How can you keep the server from logging all the requests for the images that happen when such a local page is accessed?
The SetEnvIfNoCase will set an environment variable if the page that linked to the image is from the www.example.com site (obviously you should replace that site name with your own) and the request is for a GIF, PNG, or JPEG image.
The CustomLog directive will log all requests that do not have that environment variable set, i.e., everything except requests for images that come from links on your own pages.
This recipe only works for clients that actually report the referring page. Some people regard the URL of the referring page to be no business of anyone but themselves, and some clients permit the user to select whether to include this information or not. There are also "anonymising" sites on the Internet that act as proxies and conceal this information.