I have a PNG image that gets downscaled (correctly) to fit the width of my theme, but the resulting file size is much larger than the original image:
Downscaled version 480x360 - 675k: http://justfiveminutes.files.wordpress.com/2009/01/2009-01-22_reflections_patterns.png?w=480&h=360
Original version 1600x1200 - 150k:
http://justfiveminutes.files.wordpress.com/2009/01/2009-01-22_reflections_patterns.png
I guess you could save some bandwidth by compressing the downscaled image (I assume this only affects PNGs).
The blog I need help with is justfiveminutes.wordpress.com.