What is high resolution?
Resolution is a measure of pixels, or individual points of color, and is often used to understand the sharpness and clarity of how an image displays on a screen or prints on paper. High resolution indicates an image or document is at a resolution that meets the minimum requirements for high quality printing.
The term resolution can be measured in dots per inch the the the the the (DPI) or pixels per inch (PPI). These measurements are a count of the number of pixels of color information available in an inch of space. The more pixels, the better the image quality. Below is an illustration of how the same image might appear at different pixel resolutions.
Resolution for Print
To ensure your printed projects looks sharp, clear and high quality, your art files need to be high resolution. High resolution for print is defined as 300-350 dots per inch (DPI) at the final output size. Files with resolutions higher than 350 dpi are ok to send for print, but not necessary, and the higher resolution will not improve the printed quality of your project.
Increasing Image Size
A pixel resolution of a 350 dpi image will be sharp at 100% size, but if that same image is increased in size to print larger it will lose its sharpness because the same number of pixels are being spread out over a larger amount of space. Images can be decreased in size, but never increased without losing quality.
You cannot simply convert a low resolution photo to a high resolution image by increasing the DPI in your imaging program. The printed result will be a blurry image.