.htaccess files are read on every request.
You should avoid using .htaccess files completely if you have access to httpd main server config file. Using .htaccess files slows down your Apache http server. Any directive that you can include in a .htaccess file is better set in a Directory block, as it will have the same effect with better performance.
The following lines should be included in httpd.conf to keep requests for images from appearing in the access log. It can be easily modified to prevent logging of particular directories, or to prevent logging of requests coming from particular hosts.
SetEnvIf Request_URI \.gif image-request SetEnvIf Request_URI \.jpg image-request SetEnvIf Request_URI \.png image-request CustomLog logs/access_log env=!image-request