Images do not render in Classic email templates

If we want to access the images within an email template, we have to use documents. This is done by uploading the images to the 'Documents' tab and then referencing the copy of the image on our server.

For example:
https://yourInstance.salesforce.com/servlet/servlet.ImageServer?id=document_id&oid=Org_id

Note: If images are not rendering in email template by using documents then we need to confirm below points:

1. 'Externally Available Image' should be marked on the documents.
2. Validate the document ID and organization ID on the image URL and correct them.  

Reference:
Add Images to Email Templates in Salesforce Classic   
Support Limitations with 'HTML' Templates