RewriteCond and Alias

Question

I have defined alias that looks like this:

Alias /pictures/sm/ /var/www/my_site/data/_active_thumbnails/

Later in the VirtualHost section have:

DocumentRoot    /var/www/my_site/sites/www.my_site.com/htdocs

RewriteCond %{REQUEST_FILENAME} !-f
RewriteRule ^/thumbnails/(.*)\.(jpg|JPG)    /images/stg-list-img.png [PT,L]

What I'm trying to do is to display /images/stg-list-img.png placeholder image only if the original image does not exist on the drive.

Right now it's replacing all the images from /thumbnails/. It looks like the RewriteCond is not aware about the Alias. Is there the way to overcome it?

Thanks