Personally, I would fix this in the .htaccess file. You should have access to that.
Define your CSS URL as such:
url(/image_dir/image.png);
In your .htacess file, put:
Options +FollowSymLinks
RewriteEngine On
RewriteRule ^image_dir/(.*) subdir/images/$1
or
RewriteRule ^image_dir/(.*) images/$1
depending on the site.