Do you want to use ICONs while posting a system message? Here is the tip to make it work.
Having an icon like warning/alarm will make the system message more attractive, for example, the same can be done by having the icon code inside the message.
To get to know the icon codes, go to SE16 - ICON -> Download the list into an excel sheet to get the code that you may want to use it in the system message.
Once you have the code with you, post the message text with the code attached to it, for example, @1D@ System will be down in another few minutes time, please log off.
Thanks
PrakashP