You have the ability to replace an image after an email has been sent. This will replace the image in ALL sent emails. For example, if you have used an image in 2 separate emails, and choose to replace it, the new image will appear in both separate emails.
Go to PoliteMail > Content > Images.
Find the image you want to replace and click on it to highlight it. Go to Actions > Replace. Navigate to the new image and click OK. The old image is now replaced with the new one on all sent emails, whether they have been opened or not.
In the Image library, the old image will still be visible. You need to close out of the Image library and open it back again for it to update to the new image.
Replacing images follows the rules of Group Sharing and PoliteMail user roles. For example, an Admin user would be able to replace any image. A user would only be able to replace an image they own, or one that is Unassigned.
The new image must match the width of the image you are replacing. Otherwise, it will appear stretched.