I'd like to “hide” an image on a card in Trello—not delete it—, since it has been part of the project, but I don't need it currently. So a “hide image” option will be really useful!
Unfortunately you cannot hide an image attached to a Card.
You can remove (delete) an image from a card and you can opt to not use an image as a cover (visible without opening the card), the image remains attached to the card.
To deselect a cover, open the card, and click the "Remove Cover" link next to the image that is currently the cover (it has a blue border).