300dpi will be good enough for a good quality print with no jaggedness. The magazine should be able to provide you with their submission standards, though.

To determine the pixel dimensions required for your submission you need to know at what size it will be printed (in inches) and then multiply that by 300. So, for example, if it will be printed 3 inches wide, then your horizontal dimension needs to be 900 pixels. 3000x3000 would allow for prints up to 10 inches in both axes.

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