It shows the original pics, only stretched to 800px, right? It kinda works, but the full scale pics are still loaded every time. Also, normal vertical page scrolling slows down when there are stretched images in sight.
However, properly downscaled display copies would require storing the temporary image somewhere. I don't know if that's feasible at all.
Does this system break manually set url tags? On most forums I prefer doing my own [ur=full.jpg][im]small.jpg[/im][/ur] linking. Some of these auto-linkers break it, even if the carefully created small.jpg would fit nicely.
Of course, I could test it myself...
There's hardly any silver bullet to these problems. The easiest solution would be doing nothing at all, but there are always less technical people who can't quite figure out why posting a 3000px image is a bad idea and how to circumvent it. The second best solution might be detecting if the pic is "large" and whether it already has url-tags around. Then only resize/link the plain large ones. That probably takes a bit of coding, though.
I'm rambling.