Those jaggies aren't from image compression. Image compression might dirty up an image, but it doesn't somehow alias the tower cables in the background, or make the green reflection shaders devoid of supersampling.
Frankie mentioned (I think) that they would sometimes use the screencap feature to take the multiplayer pics, and that it adds extra anti-aliasing. If Halo 3 actually has that much AA when running, it'll look great, but if it's just as jaggy as it was in the beta, then a lot of the smooth look and detail is lost, and it ends up looking mediocre again.
So, can anyone here confirm that the game is as anti-aliased as the screens make it out to be?