Could you provide a link to your site?
What version are you running?
Are you running your site through a Content Delivery Network (CDN)?
Does your webhost cache any on site resources?
Do you have a .htaccess or httpd.conf rule that instructs browsers to cache images? such as:
<FilesMatch "\.(flv|gif|jpg|jpeg|png|ico)$">
Header set Cache-Control "max-age=2592000"
</FilesMatch>