Rewrite only if Image don't exists with .htaccess

Question

How do I make RewriteCond+RewriteRule change site1.com/folder1 to site2.com/folder1, only if file in this directory don't exist. For example: site1.com/wp-content/uploads/2014/03/image.jpg

If image site1.com/wp-content/uploads/2014/03/image.jpg don't exists, than get site2.com/wp-content/uploads/2014/03/image.jpg But if image exists, don't do anything.

Update, here code what i need, thanks all for help )

Options +FollowSymLinks
RewriteEngine ON
RewriteCond %{REQUEST_URI} "/uploads/"
RewriteCond %{REQUEST_FILENAME} .*\.(jpeg|jpg|gif|png)$
RewriteCond %{REQUEST_FILENAME} !-f
RewriteRule (.+) http://site2.com/$1 [L,R=301]

Solution

Try something like this.

RewriteCond %{REQUEST_FILE} !-f RewriteRule ^(.*)$ http://site2.com/$1 [QSA,R,L]

Comments

[Ladadadada]

Do you want a solution for just the example you have given or for any image on site1.com?