For pixel-based images to print sharp and clean, they should be at least 300 dpi (dots per inch), but not too much larger otherwise they drastically increase the file size. The only exception to this are bitmap images which should be a minimum of 600 dpi. Images pulled from the web are 72 dpi which are ideal for viewing on screen, but terrible for printing. A low resolution image adjusted to 300 dpi will not fix the problem, the image will still print poorly. Scanning, photographing or downloading high resolution images to work with is the only solution for high quality printing. Even a high resolution image will deteriorate when scaled up to a larger size. Whenever possible, consider using vector-based images which have no image deterioration at any size.
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