I wasn't sure if this would be "desired" functionality for all images so I didn't modify img tags to do this, I created new ones. As you can see it also automatically links the larger version of the image, meaning that you only need to do:
[img]image url here[/img]
and the user will see the shrunk version with the option to click to see the full.
- Embeds image at 100% width
- Shrinks image to a maximum width of 600px
- Automatically links to the larger version