We recommend all files be at least 300 dpi. Please note that you can not just change the file resolution. This would be considered up-sampling and will blur your artwork. If your file is not at least 300 dpi, then it will need to be recreated.
If you need more information, have additional questions or just can't find the answer to your question here, please submit a support ticket and one of our account managers will be happy to help.