Apache ProxyPass Missing Images

The code for the forum has the /forum/ path hardcoded in the HTML that it's sending to the client, or otherwise defined somewhere in its configuration. If you can change it to /, do so; otherwise, you can work around it in your Apache config:

<VirtualHost *:80>
    ServerName forum.mydomain.com
    ProxyPass /forum/ http://127.0.0.1:8080/forum/
    ProxyPassReverse /forum/ http://127.0.0.1:8080/forum/
    ProxyPass / http://127.0.0.1:8080/forum/
    ProxyPassReverse / http://127.0.0.1:8080/forum/
</VirtualHost>

You can fixed it by placing the rewrite engine in the virtualhost.

RewriteEngine on
RewriteRule ^/forum/(images|javascripts|stylesheets)(.*) /forum/$1$2

For futher details refer here for rewrite engine