Here's a screenshot...
I generally run a separate window, on Xubuntu 10.04. But I restarted with it embedded -- same thing. I went back to separate window to take the screenshot.
Look at this book that has a cover at exactly 600x800 -- compare it to the book information cover image. (You can find this book here on MobileRead...) You'll see that the image in the browser is being squashed into a narrower ratio i.e. making it skinnier. It makes the lettering jagged, if you look closely. The book info image is fine, no matter what size it is scaled to (I scaled it here to have the same height as the cover browser.)
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