Improve "full help", missing documentation for "Picture" option under System: Settings: General

[MagicFab]

In the web GUI under System: Settings: General there is a Picture setting - URL : https://192.168.1.1/system_general.php

• The "full help" for this setting doesn't indicate a recommended format, doesn't indicate its purpose.
• This configuration option is not documented at https://docs.opnsense.org/manual/settingsmenu.html

I am using OPNsense 25.7.10-amd64