Felinik points to a site and site points to herebelow summary. 72 dpi is important for text not image.

The 72 or 96 dpi numbers are NEVER used to show images on the monitor screen. There is no concept of inches or dpi in the video system. Those logical inch dpi numbers are only used as a crude approximation to size text fonts on the screen. The size results are not very accurate, but we have nothing better for text fonts. These numbers have absolutely NOTHING to do with showing images on any screen, no way, no how, not then, not now, no matter how many times you hear others that don't understand it tell you otherwise (they are also victims, and are just parroting what they heard too). Images already have a size in pixels (the 412x324 pixel images above), and the video system simply shows those pixels directly, one for one. Video only shows pixels. This is the easiest possible result. Don't make it be hard, the correct way is much easier. If you show a 412x324 pixel image on the screen, you will see 412x324 pixels. That's all there is to it, how things actually work, the only theory that will allow us to predict accurately what will actually happen. You can clearly see this is true if you will just look at what actually happens on the screen. Dpi is not a factor on the screen, and this is one of the most basic and necessary fundamentals of digital images.

Reality goes much better if we forget about 72 dpi, and instead simply create the number of pixels that we want to see on the screen.