.htaccess Missing Image Replacement

I have been fiddling with an .htaccess file trying to get it to show a default "Image Not Available" image in place of a 404 error. I have found code in other forums that has worked for others, but does nothing for me:

Code:
Options +FollowSymLinks
RewriteEngine on
RewriteCond %{DOCUMENT_ROOT}%{REQUEST_URI}!-f
RewriteCond %{REQUEST_URI} (.*).(gif|jpg|jpeg|png)$
RewriteRule ^(.*) noimage.gif
and this one which several people said worked for them

Code:
RewriteCond %{ENV:ERROR404} .(jpg|gif|png)$ [NC]
RewriteRule ^.*$ noimage.gif [L]
Anyone had experience with this? The webserver is Apache on CentOS 5.