Yes, just as side note the CID is defined as Content-ID in
RFC 2111
It works a little different depending on what you automate, but the general idea here is the image URLs you use with cid: prefix locate their target file in the mime attachments instead of the internet. Whatever you tried earlier most probably failed, when you used file names or file URLs, URLs to internet resources will also show a logo but will be blocked as they are both signaling to the server serving the image, when a mail requesting the logo is opened. This alone is against some privacy laws and of course, you could even get more info out of such mail client requests of your server/homepage files by individualizing the URLs.
A typical message of the Outlook mail client looks like this:
There are setting in mail clients preventing to show images coming from external URLs and not embedded within the mail, there are settings letting clients always only see the text-only version of a mail. You can't prevent that, but test sending your mail to an account you configure to show images, even the simpler way to embed images using URLs to public images would work. The user needs to want to see emails with images anyway.
There's much more to it if you care for your emails not being categorized as spam, for example. But that's beyond just the image topic.
Bye, Olaf.
