1/ You can add DVD cover (poster) when you click on "Add poster" button on the images/poster page, just don't forget to read our image guidelines here.

2/ Only way to change the default poster/backdrop is by users voting for it (except for rare cases when the mods are forced to lock the default poster/backdrop manually).

3/ Only mods can delete images, so if you need to do so, fill the content report with information what image should be removed and why.