Best practices for printing clean images
DPI stands for "dots per inch" and is the standard measure of printer resolution for digital imagery. Specifically, DPI refers to the number of dots of ink or toner that will appear within one square inch of the image when it's printed.
At CustomInk, the minimum DPI required to screen print raster images - like photographs - is 200 at actual print size. For our digital printing service, the minimum is 72 DPI for dark ink on a light shirt, and 120 DPI for light ink on a dark shirt, at actual print size. The reason that DPI requirements are lower for digital printing is because digital printing goes straight from the digital image onto a shirt, whereas screen printing goes from a digital image to a stencil-like screen to a shirt, kind of like making a photocopy of a photocopy. We need extra resolution to ensure the integrity of your final screenprinted image as it moves through this three-part process.
Having trouble figuring out your image's DPI? On a PC, you can find this number by going to Start » Programs » Accessories » Paint, opening your image, and clicking on "Attributes" under the "Image" menu. If you have a Mac, open your image in Preview (your default image viewing program), and go to Tools » Inspector » Image DPI.