Or if you want to use a RewriteRule add this to your webserver roots .htaccess. This checks if the Requested filename is not a physical file or directory. if it's not. it redirects to your /app directory
<IfModule mod_rewrite.c>
RewriteEngine On
RewriteCond %{REQUEST_FILENAME} !-f
RewriteCond %{REQUEST_FILENAME} !-d
RewriteRule ^/images/(.*) /app1/images/$1 [L,QSA]
</IfModule>