To get the best-looking printed image, use a vector file (like a PDF, AI, or EPS) that's in CMYK color and has at least 150 dots per inch. This will ensure your image looks sharp and clear, no matter how big or small it is.
What does all of that mean?
HOW TO FIND DPI
This is the number of dots per inch in the image. A higher dpi means more dots, which makes the image look sharper.
On Windows:
On Mac:
In image editing software (like Photoshop):