If you dropped a fiver in the street, would you be surprised if it wasn't there when you went back?
But there's a difference with lifting images from the web - someone has picked up your 'fiver' but actually you haven't lost anything.
Is that still theft? I can see both sides, but when this sort of thing happens (and it's happened to me a few times) the person doing the 'stealing' doesn't know or think they've done anything wrong.
Unfortunately, it's not always as innocent and inconsequential as that, but finding a form of law that can distinguish between the two has evaded our greatest legal brains so far. If you don't want your images to be 'shared' in this way, don't drop them in the street.