At full size, anything you copy with a screen-capture software will have whatever resolution your screen displays. Typically, that's only around 96-120ppi. That's hardly what anyone would call 'best quality'. Rescaling the image won't improve its quality - you may not notice how poor its quality is, though, if you reduce it to half the original size or less; indeed, the 'apparent' quality may increase. If you want to insert an image into a Word document and achieve all of whatever resolution that image supports, insert the actual file, not a screen-dump.

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Cheers,
Paul Edstein
[Fmr MS MVP - Word]