You can add the following in a .htaccess in the image directory:

RewriteEngine on

RewriteRule ^$ http://your-site/ [R,NC]

What the above directive does is it sends any visitors who lands in the image folder to your site in a redirect. But it does not affect viewing of other files located in this directory - if they call the file by name.

All the above presupposes that you are on a *nix /Apache server.

You don;t need to keep an index.htm in this case.