You have a basic misunderstanding of what happens when you attach a template to a document.
Some programs like PowerPoint make other changes, but in Word the structure and content of a document are only influenced by the template when the document is created. Changing the template does not change those.
If your image were an AutoText entry in a template and it was in the document using an AutoText field, changing the template containing the AutoText would change the image. I do not recommend this.
As a lawyer, I have a great misapprehension when it comes to changing existing documents. If a document was used, I like to leave it as it was used for historical purposes. My preference, a very strong preference, is to always base new documents on templates.
A change to the template can then change future documents but not archival ones.
I have letterhead that has a logo. It has different officers on the letterhead and the office may relocate. The logo, address and list of Officers/phone numbers/emails are saved in one
Global Template as
AutoText. All letter templates use those in the letterhead with AutoText fields. When a template is used to create a new document, a macro locks those in place so they will be fixed in the letters. A similar process is used for pleadings. A change to the
AutoText will change all future documents and updates all of the templates.
Here is another method of autoupdating: Letterhead System
However, that does not help a lot with your current problem.
You would need a macro to replace all of your old images with the new one.
Document Batch Processes by Graham Mayor, MVP would help with that.