To change images in the email templates, go to 'Settings' from the side menu. Click on 'Documents' and click on 'Image Assets' to expand the tab.

Tap the more options icon at the 'Actions' column and click on 'Replace'

Click on ‘Browse’ to choose the image that you want to display in your email template. Once you select the image, click on ‘Save’ to update the changes.  

Also, verify whether the image you have uploaded is properly updated.