on wordpress.com, images can be served in different sizes based on the address (query string). For example http://alecs1.files.wordpress.com/2009/05/18-jos2.jpg?w=600. I put w=600 and my image is scaled automatically to 600 pixels on width, saving load times.
This does not work on my WordPress installation however. How could I enable this? Or what keywords should I search for? I searched for "wordpress set image size in query string" and variations, but to no success.