1. Install plugin.
2. Edit the config file. config/OnScreenLogo.json
3. Delete Plugins/OnScreenLogo.cs and data/OnScreenLogo.json Leave only config/OnScreenLogo.json edited file.
4. Install plugin again. In 1 minutes image loads on your server.

Edited config file. config/OnScreenLogo.json
example :
{
"Image. Link or name of the file in the data folder": "http://imghst.co/72/eAaIfjZwp4.png",
"Maximum anchor": "0.34 0.105",
"Minimum anchor": "0.26 0.025"
}

Thank to the creator! ;)

Editing a configuation file only requires a reload of the plugin afterward, not deleting the plugin. You can reload using the commands that Oxide provides: oxide.reload OnScreenLogo

It did not work for me. This is the only thing that worked for me. 

If you say so, but they do the exact same thing.