Some of you may have noticed that photos posted to flickr don’t match quite with the original versions. Resized down automatically to create the default sized images, they are also sharpened and (questionably) have their saturation enhanced. There’s a blog entry going into more detail here.
Speaking from experience with image production for web use, I’d strongly recommend uploading photos in increments of 500px (width), so that when they’re re-sized to 500px wide by the flickr robots, the reduction is perfectly proportional. That way, any sharpening applied should be less jagged than if the image were to be re-sized from an arbitrary width.