Tell Apache to serve missing images from a remote server. Props http://stackoverflow.com/questions/5130410/modrewrite-to-another-server-if-file-image-is-not-found

.htaccess

# BEGIN serve images from production
<IfModule mod_rewrite.c>
RewriteEngine On
RewriteCond %{REQUEST_FILENAME} !-f [NC]
RewriteRule ^(.*\.(js|css|png|jpe?g|gif|ico|svg)) http://example.com/$1 [NC,P,L]
</IfModule>
# END serve images from production