Instructions
Open your "GMod" installation directory, located on your main hard drive in the "Program Files" folder.
Find the file named "motd.txt" and double click it to open it with a text editor. The message of the day information is written in html format, so be cautious when editing if you are not familiar with the code.
Add in the line "<body style="color: rgb(0, 0, 0); background-color: Black; background-image: URL(IMAGE);" and replace "IMAGE" with the location of your picture. This will embed the picture into the MOTD the next time you load "GMod."
Save and close "motd.txt" and load your "GMod" server to test the message and image.