Yes, you are able to edit the default images that are in templates and are in fact encouraged to do so. Making your newsletter resemble your business will help build customer loyalty, as they will visually make the connection between the campaign and you. This can be achieved by inserting your own images into the campaign.

You can delete default images or images that you have placed within the template.

If there is a background default image in the selected template, you have the option to delete it.

Note: Most templates restrict your ability to move or resize the default images.