We intentionally made this method not directly accessible because of the potential for abuse. We actually had a GUI ready for setting it up through server settings but decided against it. It's mainly intended for gamemodes and highly scripted servers.
Sounds more like a problem on your end than his. Why not make the player download the image from the server than from online? I can see why you say it has the potential for abuse, becuase players could link it to researchographic or gory images or gigantic image files for their server preview image, but really, isn't that for the owner of the server to decide? You can't moderate what people do on their own servers. Drama them if you wish, but don't try to tell players what server preview image they can and can't use because you may disagree with the content.
This really doesn't even qualify as an add-on. It's a single line of non-runnable code that you intend people to change. They could just as easily insert this in to the console.
I partially agree; it belongs in general discussion probably. Since it is technically not an addon, moreso a guide of how to use a feature of blockland glass