I want to allow users to click "share this" on a page with an image, and then email that image as an attachment
If you are willing to write your own module, check out the mimemail module. Sending the mail is as easy as this:

mimemail($sender, $recipient, $subject, $body, $plaintext = NULL, $headers = array(), $text = NULL, $attachments = array(), $mailkey = '')


If you store the picture in a imagefield or filefield inside a node, you can put that field into the attachments array and send it.