Exposing a document for external usage
sometimes you need to expose an image that you have to be available on site pages or autoresponders. Salesforce makes it easy to upload and expose documents.
Give it a name and check the Externally Available Checkbox
Then Right Click the preview image
And then you have the URL on the clipboard.
It looks somthing like this
is you're using the image in an autoresponder email, just remove the S from the https:// to allow older versions of outlook to display the image properly.