All non-vector artwork should be supplied at a minimum of 300 dpi. If images have been downloaded from the Internet, normal screen resolution is just 72dpi. Therefore, if the image is 5 inches across on screen, it will print approximately 1 inch in size at 300dpi.
For images smaller than actual size, increase dpi proportionately. e.g. 50% of actual size = 600dpi.
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