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Adobe Illustrator CS2 has a 'Live Trace' option which will convert any image into vector graphics. You can then resize it. Check it out. The Flash animation on this page shows a demonstration:
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If it is a single image then you could try this out:

http://www.myjanee.com/tuts/pentool2/pentool2.htm

Other than that if it's a scenery or a photograph just as the others said, you can't...