The [ resize ] [ /resize ] tags (remove spaces) wrapped around any image will automatically scale the image to 1000 pixels wide. You can wrap the tags around multiple pics (or even your whole post), and it will scale everything to 1000 pixels wide. There is an icon at the top of the editing window to automatically add the tags:
To Chris's point, that doesn't affect the bandwidth the image requires to load, so there is no benefit for those with slow connections. That said, multi-pic threads are always going to be clunky for those with slow connections, so I don't worry about it terribly. Sometimes I repost to imgur to resize, sometimes I use attachments, but if I am fixing a thread that has the pages skewing wide due to someone's mongo picture, I usually just add the resize tag as it is quick, easy, and effective for that particular problem.