This .htaccess will optimize your server and increase your page load scores (higher score is a good thing). This will set the images to expire after a certain amount of time as well as numerous other optimization tricks for your server. These set the expires headers for images, and set some cache-control headers as well. Place this at the very top of your .htaccess file, remember to back up and test first.

<ifModule mod_expires.c>
  ExpiresActive On
  ExpiresDefault "access plus 1 seconds"
  ExpiresByType text/html "access plus 1 seconds"
  ExpiresByType image/gif "access plus 2592000 seconds"
  ExpiresByType image/jpeg "access plus 2592000 seconds"
  ExpiresByType image/png "access plus 2592000 seconds"
  ExpiresByType text/css "access plus 604800 seconds"
  ExpiresByType text/javascript "access plus 216000 seconds"
  ExpiresByType application/x-javascript "access plus 216000 seconds"
</ifModule>
<ifModule mod_headers.c>
  <filesMatch "\\.(ico|pdf|flv|jpg|jpeg|png|gif|swf)$">
    Header set Cache-Control "max-age=259200, public"
  </filesMatch>
  <filesMatch "\\.(css)$">
    Header set Cache-Control "max-age=604800, public"
  </filesMatch>
  <filesMatch "\\.(js)$">
    Header set Cache-Control "max-age=21600, private"
  </filesMatch>
  <filesMatch "\\.(xml|txt)$">
    Header set Cache-Control "max-age=21600, public, must-revalidate"
  </filesMatch>
  <filesMatch "\\.(html|htm|php)$">
    Header set Cache-Control "max-age=1, private, must-revalidate"
  </filesMatch>
</ifModule>
<ifModule mod_headers.c>
  Header unset ETag
</ifModule>
FileETag None
<ifModule mod_headers.c>
  Header unset Last-Modified
</ifModule>