Here is how to setup an icon:
1. Have an image that measures 64x64 pixels and has the file name
server-icon.png. Minecraft will not accept files that do not follow this standard.
2. Login to the Modern Hosting Game Panel, select the server you want to change, and upload your image using the “File Manager” tab. Alternatively, you can upload your image using SFTP. If you are unfamiliar on how to do this, you can see this article here to learn how to upload files.
3. "Restart" your server using the button on the console page. When you go to the server list, your server will now have an icon on the left side.