In the old desktop version of sketchup you could add an image as a background from the edit styles tab. I think this was the watermark function. The background image stays in place when the camera scrolls in and out. Is the function in Sketchup for schools? I can import an image into the file but it moves with the camera. Is there some way to lock its position.
What you refer to is called a watermark. There isn’t an option in the web versions to add a watermark, though.
Your profile says you are using SketchUp 2021 Pro. You could create a model file with a watermark in the style and upload that to SketchUp for Schools. This is an ancient style with a watermark that I made.