Download from the workshop "ServerAdminTool Image Mod".
Create New Project with Enfusion Workbench, with the addon "ServerAdminTool Image Mod" included to your project. 
In your new project, add a "Images" folder. Look at the directory structure of the "ServerAdminTool Image Mod", and follow.
Go to the [HOW-TO] Create an .edds file for images and follow the instructions there. Return to step 5 below.
Publish your mod to the workshop. I believe you can publish as unlisted, and still use the copied link to the file. I will verify this and update.
Right click on YOUR .edds file created in step 4, and select "Copy Resource Name(s).
Open the "ServerAdminTools_Config.json file on the server.
﻿ Locate the "serverMessageHeaderImage" in the file and paste your copied resource name.
"serverMessageHeaderImage":"{CD45B485244FF76B}Images/IMG_0122.edds"﻿
 
Add both mods to your servers config json file. "ServerAdminTool Image Mod", and whatever project you created above.
Save the file, close, and test your server.

ServerAdminTool Image Mod

1.0.1

1.0.0