To change an image hover over it and a small pencil will appear near it. Click it and a window with 2 options will appear:
1. An icon picker button which leads you to a list of icons. Here, you have the option to change the image with an icon, and set-up its color, insert a link and manually set the width and the height. You can see more details here.
2. An image editor button. Here you will be able to add the url address of the image that you want to use. There is also a recommended size for you to use to avoid future problems. Set the width and height and an Alt tag and title tag if you want. To make the image clickable add an url to the ‘LINK URL’ area. You can see more details here.
Save it to apply the changes.