A majority of the old school surfers (what is that, maybe 40%) have 800x600 res. With windowsXP it forces a higher res desktop. So I think that bigger pictures could do better now and in the future. I prefer images that can fit on a 1024x768 screen so that they're bigger than other galleries, fit the screen of the newer PCs with the default rez, and aren't too huge for 800x600.
If a person has 800x600 res, they probably won't know how to view the picture at a smaller size if they have their picture resizing tool turned off in IE.
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