How Image Resolution Affects Print Quality - Photoshop Tutorial
So how high of a resolution value do you need for professional quality printing? The generally accepted value is 300 pixels/inch. Printing an image at a resolution of 300 pixels/inch squeezes the pixels in close enough together to keep everything looking sharp. In fact, 300 is usually a bit more than you need. You can often get by with a resolution of 240 pixels/inch without noticing any loss of image quality. The professional standard, though, is 300 pixels/inch.
Let's consider these pictures from Reddit. I opened Irfan View, but it doesn't show me how many pixels/inch?