Well, I don't think you quite understand what I'm getting at here. There are forum settings that automatically resize each image to fit-to-screen (and then give you the option to show the image at the original size). That way no member has to consciously resize each image to certain sizes.
Take a look at this forum page:
http://www.northamericanmotoring.com...=93550&page=65
You'll note that all the images on the page are huge but once the images have been loaded, the forum resizes the images itself so that each page is easily viewable.