Limiting user access to certain features

I post a lot of photographs on my blog. I want to give some users the ability to email those pics to whomever they please.

Since anyone can use the form to send email, I have an open relay right now for spammers to use…. That is not a good thing.

Right now, access is limited only via the referrer mechanism, which is not really secure. I would like to add the following:

1. Limit access to the form only to registered WP users above a certain level

Any way I can pass user info around so I can be sure the requestor is registered and get WP to provide the user email to me?

2. Limit the POST method via WebDAV (this is more of a PHP question.)

Any way to get WP/PHP to authenticate to apache via WebDAV?

The form is not really a part of WP; it pops up in a popup window so it doesn’t have any of the WP window dressing or themes. I can pass whatever cookies, etc. I need but I have no idea even where to start….

I’ve looked at the other email/form submission plugins, but they really are more for user comments to the author than for forwarding images from the blog to friends.