I downsize all my images just out of habit when I post anything I've taken because my camera is taking them at RAW 5000 pixels wide basically so that would piss a lot of people off (not to mention have me uploading something like 20mb images).
I try to drop them to around 800 wide at most. If I want people to have access to a larger image I just URL it. So URL the larger image around the smaller image so they can just click it to get the bigger one.
Truth is (and I've done a lot of web design and GUI interfaces), you can't design for full screen. Even if "most" people run 13something wide, I doubt "most" people run their browse maximized all the time. I for example RARELY have it maximized. I have outlook maximized on 1 monitor and I have about 10 IE/Firefox/etc. browsers going all at various non max sizes scattered around between programs and such. So when I view the forums my window is usually much smaller than max resolution.
Ultimately though I just deal with it when people post images or link ones massively huge and in my many many years on many different forums, I think it is ultimately a losing cause unless moderators become extremely militant about it and police it to the nth degree which is difficult to do because they have other stuff to do and deal with far more destructive on forums. It's kind of like people turning left on red lights. A lot of people do it and it is amazingly illegal, but the Police say they'd rather go after actual criminals that cause public issues than sit their going after every single red light turner. I don't 100% agree but I can see their point of pick you battles.
Too bad we don't have an automated way of having the system scan an image being posted and go "Hey wait sorry this is too big try again."
Kind of like your red light camera.
For the record the PC I'm on righ tnow is set at 1680 x 1050 and I have 2 x 22" monitors (this is their maximium resolution, and they are standard Dell business LCD screens). At home I have my PC on a 32 inch LCD TV which is running 1366 x 768 and at most if I flip my PC over to my 60" TV in the living room, that runs 1920 x 1080 so not talking massively huge resolutions. I think images of at most 1027 x 768 would be fine by me. Anything larger and it starts to become an issue. And like I mentioned, I try to post images only at 800 x 600 because I know not everyone browses full screen and let's face it, you typically don't need images larger.