It wouldn't have been available at the time you asked this question, but if you're using Apache HTTPD 2.4 or later, then as described in the docs you could do it with two location blocks:
<VirtualHost *:80>
...
<Location /site>
ProxyPass http://server.local:8989/site
ProxyPassReverse http://server.local:8989/site
</Location>
<Location /site/IMAGES>
ProxyPass "!"
</Location>
</VirtualHost>
Location blocks are interpreted in order, so you need to put the no-proxy one after the proxy one. Alternately, if you want to keep the proxy directives at the vhost level, the answer given by fission works just fine!