Imagine that computer RAM or a classic hard-drive are like a bunch of tiles an Othello/Reversi game board.

When you delete a picture, only the tiles that say "photo.jpg is inside tiles X to Y" are erased and flipped to be white-side up.

The actual tiles in spaces X to Y that had all of the pixels are usually left untouched, unless we have a paranoid reason to go through and change them.

In no case are any tiles being removed from the table--that would represent damage to the system.

Later those X-Y tiles may be changed, but usually because we've decided to put something new there.