If you're using Apache, you can add expiry information in your .htaccess file. For example:
# Enable the mod_expires module
# Set expiration date to 1 month for all style sheets and images
ExpiresByType text/css "access plus 1 month"
ExpiresByType image/jpeg "access plus 1 month"
ExpiresByType image/gif "access plus 1 month"
ExpiresByType image/png "access plus 1 month"
# Set expiration date to 1 week for all HTML pages
ExpiresByType text/html "access plus 1 week"
ExpiresByType application/xhtml+xml "access plus 1 week"
What time-scale you choose depends on how likely you are to amend the image. The expiry is basically saying it's OK to serve a cached copy of this resource for another day/week/month or whatever after it's been accessed. For images, it's probably safe to set a lengthy expiry, because if you change the image, you're likely to replace it with one with a different file name i.e. a completely different resource.
There's a good tutorial here that you might find useful.