nCode Image Resizer works
really well. The only thing it doesn't do is resize the image before it's sent to the browser but at least it doesn't break the widths of your threads
As far as I can tell there isn't any general vB option to control the size of images posted inside [IMG] tags. If you want to physically resize images, there's something called phpThumbs which does it on the server for you. Working with remote images is tricky though.