Clipart is usually in WMF format, which is a set of drawing commands. When
you enlarge it, you are telling the command to "draw it bigger." Photos, on
the other hand, are made up of pixels, which are little colored dots. There
are only so many of them in a photo, and if you make the photo bigger, the
same number of pixels gets spread out over a larger area, so you begin seeing
the individual dots.
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Ted