To change/replace an image on your website template that is in an Gallery, double click on the image that you would like to change/replace and select Edit Item Content from the list.
Now you will need to click on image that is shown on the Gallery Menu, you will not see the option to Change the image.
If you would like to change/replace an image on your website, you can do this by selecting the image and clicking on Change Image from the Image settings panel.