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I think maybe my last post wasn't clear. 1024x768 is average monitor resolution. 800x600 and 640x480 are much smaller than 1024x768 and should allow most viewers to see good quality pics and not have to scroll sideways. If you made the pics 1600x900 they'd be GIANT on most screens and the majority of folks would have to scroll sideways to see it all.
The first pic you posted is a good size at 576x431 but you can get away with a bit larger.

Does that make sense?

640x480 and 800x600 are good pic sizes for the web. I believe the average monitors default resolution is 1024x768 these days so you definitely want the pics to be smaller to avoid making people scroll sideways.