Which is why usually you'll see things like watermarks -- one of the BEST watermark methods is to put the images behind a user login (assuming you have some sort of subscription service) then encode the visitors IP and account name into the image file in realtime. Takes a bit of server power to do, but it means when someone grabs the image and uses it somewhere else, you at least have a CHANCE at figuring out who took it and ban their account...
... but that's mostly only useful AFTER they've leeched you dry, to protect new content, not the existing stuff.
Bottom line, HTML just doesn't work that way; if you don't want people leeching your images, don't put them online. That's harsh, but it's also the reality of it.