.htaccess rule

We are having an issue writing a .htaccess rewrite rule for a specific case. The situation goes like this: - We have an apache alias called /imgc - The same /imgc will be accessed from multiple different sites on the same server. The reason why we chose to create it as a common alias is because I wanted it reachable from all the sites. - imgc can have many folders and each of the folders can have multiple image - rewrite rule is supposed to be written so that when the image is requested and not found in any of the folders under /imgc the user should be redirected to a different full url something like [login to view URL]{image requested} We need help writing this .htaccess file. It should be a fairly simple job for someone good with rewrite rules. Thank you