In order to change the network picture, please go to the "Network" tab from the menu which is on the left side the left and select the network and click on it. You will see the name of your network, regions, gateways, tunnels, access groups, and tags. At the bottom of the bar on the right side of the screen, there will be a "three dots in a circle" icon, please click on it. You will see "Edit Network" option, please click on it.
Here you can edit the network picture by clicking "Browse". After changing the picture, please click "Save" and your new network picture will be saved.