What you're describing doesn't degrade the image. In fact not a single picture is changed. What it does is change a flag in the drawings data that's just says "1 inch of this picture will contain 300 pixels (dots) when printing". Which just means that if your photo has a resolution of 3000x2000 pixels the resulting printing size equals (3000/300 = )10" by (2000/300 =) 6.6"

What I think iStockphoto means is don't upsize your picture from 3000x2000 to 6000x4000 for example because you'll just be adding data. In other words you're creating pixels out of thin air.

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