Q: I have changed the image in an HTML signature but apple mail is still showing the old image

I have updated an image for our email banner that runs along the bottom of our HTML email signature (the file name is the same and it is sitting in the server under the same file extention).

Although it shows up with the new image on my iphone and also to other people I send the email, in apple mail on my macbook it still has the previous image that was in the signature.

As I know that it works (as I can see the new image when it is sent to someone else) why does it not update on my own macbook?

Is there a way of refreshing apple mail so I can see the new image?

Any help would be appreciated.