I personally recommend that you resize an image yourself or use a site such as Flickr which gives you the image at a few standard denominations smaller than what you give it. I believe one of them typically ends up at 320 or 360px wide.

We may or may not currently have something rigged up to limit the display width of an image to 400px, but that doesn't mean when you load a post with images, you aren't loading a picture that's a thousand or more pixels wide, just as a warning.