When preparing files for Print on Demand, we recommend using Save As (or Save a Copy) rather than "Save for Web (Legacy)." The Save for Web tool is optimized for screens, meaning it prioritizes small file sizes and fast loading times over the high resolution required for a crisp print. Using this tool often reduces your file to 72 DPI, which can lead to grainy results.
How to save for the best results:
In Photoshop: Use File → Save a Copy and choose JPEG or PNG. Ensure your resolution is set to at least 300 DPI under Image → Image Size before you save.
In Illustrator: Use File → Save As and select Adobe PDF. Choose the "High Quality Print" preset to ensure your vector lines stay perfectly sharp.