I've uploaded an image file through SF File Upload process, used the Copy Link option and inserted into my email signature, however it is not showing in my emails.  I have not problem going directly to the link and viewing the same image file in my browser session.  I've also been able to upload the same image file to an external server and the image displays properly in the email signature.  Here's the html code for displaying the image:

<img src="https://ciopartners.my.salesforce.com/sfc/p/6A000000tGyz/a/6A000000fy0h/NhVH4puAGXo7K1rOhbbWJ4w1jaNOQ1ds6wf5hFFULxg" alt="Company Logo" />

Here you can see that the image is not found by SF when creating the email:



TIA,
Mitch