I think this only works with screenshots, you can Move them up, Down and Delete.
But when I want to place some image in a game Description I click on Add Image where I can Upload or Choose Image. If I decide to replace an image, older ones are left in this "Choose Image library".
I've tried this too - if I place the image in the Description and click on it I can change its name... etc, and Delete it. But when I delete the image it is deleted from the Description and not from the "Choose library". It stays there for good.