When working on large WordPress sites that have a very big uploads directory that is regularly updated and added to. I did not want to have to download the content via FTP every-time I wanted to work on the site locally.
I used the following rewrite rule, placed in a .htaccess file in my wp-content/uploads folder.
<IfModule mod_rewrite.c>
RewriteEngine On
RewriteBase /wp-content/uploads/
RewriteCond %{REQUEST_FILENAME} !-f
RewriteCond %{REQUEST_FILENAME} !-d
RewriteRule ^(.*) http://your-site.com/wp-content/uploads/$1 [L,P]
</IfModule>
This looks to see if a file exists locally if not it shows a remote one.
You need to make sure you set both the RewriteBase (line 3) and RewriteRule (line 6) to match your environment.
Source: http://polevaultweb.com/2014/03/5-ways-synchronise-wordpress-uploads-across-environments/
Multiple States Knowledge Base 
Just note that because of empty line ‘line 3’ is actually line 4. Similarly, ‘line 6’ is line 7.
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